It is the number of the Beast in Revelation:

Revelation 13:18 (ESV) ^[1]
18 This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666.

Its significance is often claimed to be that the Emperor Nero, during whose reign Revelation was written, has a name with the numerical value 666. Citing Wikipedia: ^[2]

The Greek spelling, "Nerōn Kaisar", transliterates into Aramaic as "נרון קסר", nrwn qsr. The Aramaic spelling is attested in a scroll from Murabba'at dated to "the second year of emperor Nero." Adding the corresponding values yields 666.

Personally, I'm not really satisfied with this explanation at all and am waiting for other answers.

In Revelation 13:18 ^[1], as cited by Dancek, it is the number of "the Beast".

The verse indicates that is the number of man as well. A possible reason for this is that mankind was created on the 6th day, so 666 is a three-fold repetition of that. I understand in Hebrew that repetition signifies degree, so God is called "holy, holy, holy" in Isaiah 6 ^[2].

666 has several possible meanings. The most prominent answer is that, as it is the "number of a name" that it would be Nero. Since there are early manuscripts which have the number as 616 instead and this also can reference Nero (I believe it is in Aramaic as opposed to the Greek).

There are others who have stated that it has to do with the fact that it is DCLXVI in Roman numerals, something which is supposed to have been of special note in ancient Rome.

Personally, I find the Nero answer the most convincing with a caveat -- it is the symbolic future Nero. Most Biblical numerology, in my reading, is similarly future driven (and ALL Apacolyptic literature of that era -- Daniel, Revelation, Enoch, Hermas) -- it has a historical meaning, with an implication for the future.

Understanding this number is deeply rooted into how one understands Revelation. I understand it as speaking of a near prophecy of the Roman Empires attack on the church, and in so doing, mirroring a continuous and distant prophecy of the churches existence in the world until the final judgment, with some things principally intending their final fulfilment. In this contact 666 probably meant something very specific to the readers at the time (which they could decode) as well as being symbolic of something for all time and which at the end of the world may also take on renewed and final significance. History in the Bible is treated as a kind of prophecy so the meaning of 666 may be fulfilled more than once.

The initial meaning has been proposed by those who have studied gematria. Gematria was a popular practice at that time which corresponds letters to numbers, so that summing up a name gives a final number. This was practiced by Jewish mystics. For example, it has been argued by scholars that when the name "Nero Caesar" is spelled with Hebrew letters as נרון קסר (NRWN QSR) — Hebrew is written without vowels — the following addition is made from left to right: 50 + 200 + 6 + 50 + 100 + 60 + 200 = 666.

Emperor Nero may very well have been the initial meaning to the readers. Although our certainty about this is scanty for it seems no standard traditional view developed in subsequent church history, it nevertheless points to the intended meaning. There is little question in my own mind that the beast which is represented by the number was Rome. Rome especially as it represented Emperor Worship, making it difficult for Christians to engage in trade, without somehow paying homage to the idolatry of Rome, was ‘the beast’ opposing the church. Not only so but this political system that opposed Christ and enforced idolatry was soon to breakout into full undisciplined persecution against the church, which John wanted to prepare them for.

This naturally leads to a higher symbolic meaning of the number of 666. For it is more than coincidental that the Emperor’s, who represented the fullness of the ‘sinful nature’ in a political system opposed to God, should have a name that is also equivalent to the plain biblical meaning of man-man-man. As man was made on the sixth day, this number is connected to him from the earliest scriptural history. Furthermore as seven represented perfection and holiness, being that this day was made holy, with the opposing facts that man fell and became unholy, he fell short of holiness. Therefore the number means ‘unholy-unholy-unholy’ or ‘fallen short-fallen-short-fallen short’. Three, is often meant to mean perfection in the Bible so this ‘sinful nature’ described by six, is said to be a perfectly sinful nature by identifying its fallen state three times.

We are now is apposition to see the meaning of 666 in its wider sense. The Roman Empire was a beast, the first beast that came out of the sea (Rev 13:1). The second beast that came from the earth was the religion of Rome in its emperor worship, having ‘all the authority of the first beast ‘ and made ‘the earth and its inhabitants worship the first beast’(Rev 13:12). By enforcing idolatry among the various professional guilds and trades in the empire it became more and more impossible for Christians to earn a living without worship the beast. They must be prepared to be martyred according to the prophecy.

Those who fell into the idolatry of Rome and denied their faith had the mark of the beast on their head, or hand. (Rev 13:16). This implies that if a Christian lived a wicked life, denying Christ and falling under the power of wicked Rome they were effective publishing their sins in an obvious way, on their foreheads, or in a more secret manner on their hands. Another view may be that their minds and actions were rules by the sinful nature.

As history progressed the world has continued to persecute the church with the same pressures of idolatry, whether the pressure to worship sex, career, religion, etc. Also at certain points Rome itself has been interpreted by many Protestant theologians as taken a heightened biblical role again. According to these view, Roman history shows a kind of re-fulfilling of its original role, making people worship saints and other idolatries, even killing Christians in the same manner as the original fulfilment. It makes one wonder whether Rome might not have some direct relationship to the events marking in the end of the world as well. However, regardless of one’s view this can’t be limited to any one government but represent all secular governments and all the idolatrous nature of man.

Mike talked about this, but let me clarify a little:

In Greek and Hebrew, they use the same symbols for numbers that they use for letters. Basically, they use the first nine letters of the alphabet for the numbers 1 to 9, the next line letters for 10 to 90, and nine more letters for 100 to 900. As neither Greek nor Hebrew has 27 letters in their alphabet, they used alternate forms of letters or extra symbols to pad it out. See, for example: Hebrew Numbers ^[1] for how it's done in Hebrew. Then for the number, say, 123, they would use the symbols/letters for 100, 20, and 3.

It would be like in English if we used A=1, B=2, C=3, ... I=9, J=10, K=20, ... R=90, S=100, T=200, ... Z=800, and we'd have to make something up for 900. So the number 123 would be written SKC.

Note that this wasn't some little side game people played now and then. This was how they always wrote numbers. So any number you wrote could look like a word, and any word could look like a number. You had to tell which it was from context.

Also note that unlike Hindu-Arabic numbers, where the order is significant so 19 is a very different number from 91, in Greek and Hebrew the order doesn't matter. The Jews normally wrote with the symbol for the biggest number first, but it wasn't essential. (I'm not sure about the Greeks.)

So they couldn't help but see numbers in every word they wrote. So let me go back to my comparative English example. (As neither I nor, I suspect, many others on this site know Greek or Hebrew.) So for example if you saw the word GUN, you would naturally think 7+300+50=357. You couldn't help but think of 357 as the "number of the gun". It would be inevitable that you'd have cases like, you'd tell a friend that you got paid $357 and he'd joke, "What, do you have a job shooting people now?" And you'd both laugh because you'd both quickly get the joke. Etc.

Of course many words wouldn't be the natural way to write numbers. Like you would normally write 123 as 100+20+3, SKC. But if you saw SKAB, i.e. 100+20+1+2, you would still add it up to 123. So any word could be read as a number, even if it wasn't how you would naturally write that number.

So when John talked about "the number of his name" being 666, this would be instantly understood by any Greek or Hebrew person as meaning that if you add up the values of the letters in his name, it will total 666.

Yes, the Hebrew version of Nero's name adds up to 666. At least, people give a Hebrew version that adds up. I freely admit that I don't know if this is the way people routinely wrote his name in Hebrew or if it is contrived. Anyway, maybe that means he's the Beast that John was talking about, maybe that's just coincidence, I don't know. I've heard so many conflicting theories about Revelation that I reserve judgement.

Let me add that there's a game that's played now and then of taking some public figure that you consider very evil and dangerous and showing that his name adds up to 666. Well, it stands to reason that most names would add up to something less then a few thousand. So one out of every few thousand names likely adds up to 666 by coincidence. With 6 billion people in the world today, if we translated all their names to Greek or Hebrew, there should be a million or so who add up to 666. Given that there are often multiple ways to translate a name between languages, and that you can plausibly choose either Greek or Hebrew, and that you could pick first name or last name or both or use initials or middle names or other variations, you probably have at least a 2 or 3% chance that any given name will come out to 666 without any stretching. Then toss in all the games that people DO play, like casually saying "... and if we double the 'r' ..." or "... and add 32 because he became a senator when he was 32 years old ...", or simply making up their own system of assinging values to letters with no reference to how the Greeks or Hebrews did it, and I'm sure you could get anyone you wanted to add up to 666.

Update in Response to Steve

As others noted, the number 666 is associated with a very evil person, the Beast of Revelation. Of course this doesn't make the number itself evil. When the number 666 comes up coincidentally, like if your change comes out to be $6.66 or a question is the 666th question posted on a web site, people will joke about this being evil. Perhaps some take this seriously. But nowhere does the Bible say that a number is good or evil per se or that such random numbers have any significance. In this case, all that the Bible says about this number is that it is the number of the Beast's name, as I discussed above.

Some people make much of "numerology", attaching some sort of mystic significance to numbers -- in the Bible or in life in general. I am not aware of any place in the Bible that endorses making a superstition out of it. Occasionally the Bible does link two numbers in a symbolic way. Like Exodus 20:11 says that, because God created the world in six days and rested on the seventh, that therefore people should work six days and rest on the seventh. Or, Matthew 12:40 says that, as Jonah spent three days in the belly of the whale or great fish, so Jesus would spend three days buried in the Earth before his resurrection. Perhaps this indicates some special significance to the numbers seven and three. Personally I think God is just making a point about a comparison between two things by re-using a number, not indicating that the number itself has significance.

I suppose a number can acquire symbolic significance just because it is the number associated with something important. Like because grass and leaves are green, the color green has become symbolic of environmentalism, we talk about "green energy" and so on. But I think few see the color green as having any sort of magical properties per se. It's just a color chosen to represent an idea.

Does it matter that the number of the Beast is 666 and not, say 523? The Bible doesn't attach a meaning to the number. Maybe that's simply what the number comes out to. Like if the police warn that a dangerous felon has escaped from prison and that he is wearing a green shirt, I don't ask what the symbolic significance of the color green is. I just take it that that is what color shirt he happens to be wearing. I've heard some discussions of why the number 666 is significant. One I recall is that 6 represents man because man was created on the 6th day, and 3 represents God because God is a Trinity, so 666 is man making himself God. Maybe, it's plausible in a way, but the Bible doesn't say this anywhere.

One of the reasons people believe the number 666 is evil because of how it is to be forced on humanity against its will in the future (if this incident takes place in the future as a literal event, which not every Christian group believes).

In Revelation 13:15-18, we find an individual leader who makes all people to take the mark in their right hand or forearm, and no one would be able to buy or sell but he who has the mark. This is commonly called the mark of the Antichrist. Modern Christian literature has made much of the fact that this marking is now possible in our digital age, when all the banks and financial centers of the world are interconnected. It is possible to have a mark, or chip of some kind, embedded under the skin that could conceivably replace physical money and cards like credit cards.

So the fear comes from the hysteria that this may happen in our day. Why? Because of the warning in Revelation 14:9-11:

And another angel, a third, followed them, saying with a loud voice, “If anyone worships the beast and its image and receives a mark on his forehead or on his hand, he also will drink the wine of God’s wrath, poured full strength into the cup of his anger, and he will be tormented with fire and sulfur in the presence of the holy angels and in the presence of the Lamb. And the smoke of their torment goes up forever and ever, and they have no rest, day or night, these worshipers of the beast and its image, and whoever receives the mark of its name.”

So people are afraid to take the mark, and equally afraid that if they don't, then they will lose their homes and jobs, for they won't be able to get paid and wouldn't be able to support themselves without the mark.

This is the reason for thinking it is evil, because of the great misfortune of those who take it, and those who don't.

The triple 6 basically signifies that the person is sealed/marked unto condemnation by the unholy trinity (i.e. the dragon, the beast, and the false prophet), just as the believers are sealed with the stamp (Rev. 3:12), which has on it written the name of God (Jehovah), the name of the city of God (New Jerusalem, the metaphor for the Holy Spirit) and the new name of God (Jesus, since He is the same Jehovah, but now just come in flesh, 1 John 4:2).

Every person gets sealed with the spirit of faith in what he/she believes for salvation (Eph. 1:13; 2 Cor. 4:13). So, for example, if I believe that an idol will save me from hell, my heart would be filled with a spirit of faith in things that cannot save me. Every myth or false doctrine can be symbolized by a 6, because they are creations and because God created everything in 6 days, so that everything created can be symbolized by a 6. Only truth, which is God, cannot be symbolized by this number 6, since God was never created (He always "was"). Faith in truth for salvation, on the other hand, would seal me with 777 on my forehead/arm, i.e. the Holy Trinity. God/truth (John 14:6) can be symbolized by a 7 because God is Eternal Life/eternal rest (John 14:6), and because the Sabbath, which is the 7th day, is a day of rest.

Note also that the same spirit (of faith) which seals us, will likewise rest in our hearts, because it is written that people "believe" by means of their heart (Rom. 10:10). So, if I am not a Christian, for example, I will have 666 in my heart. If, however, I am a Christian, I would have 677 in my heart. Not the full 777 (that's what seals me, but it's not completely what will be in my heart), but only double 7, because one place is occupied by our sin-nature. Not the sin-deeds but the sin-nature. Our sin-deeds get washed away, but our sin-nature can only be washed (not washed "away") and disinfected, like the broken skin of the beaten man by the roadside, to whom ministered the Samaritan.

Our sin-nature, however, gets washed and atoned for when Christ fills our heart to the brim (Romans 8:10). Our sin-nature is with us until we die, and all we can do now is to wash our soul from all past sin-deeds, and get clothed with the double garments of the Spirit, i.e. get filled with the Spirit to the brim. Christ will fully heal us from it only when we meet Him on the other side. It is here to keep us humble by preventing us from exalting ourselves into thinking that we are the sinless God (1 John 1:8; 2 Corinthians 12:7-8).

Note that guilt is like hot coals, it quickly burns holes in our wineskin (Lev. 5:3), causing the wine to spill out and not be able to fill us anymore. This is what we should be aware of most, because if we keep our conscience clear, it is very easy to get refilled, since everything we ever do or say comes from our heart (Matthew 15:11; John 10:38), yet nothing can come out from our heart unless it fills it to the brim first (Luke 6:45; Psalm 23:5)! That means that whenever we do or say something from the Gospel, which can be only done by the Spirit (Luke 6:35-36; 1 Corinthians 12:3; John 15:5; 1 Peter 1:21), we get filled with the Spirit!

As for what the beast is, or the dragon, or the false prophet, the false prophet is everyone who represents those who preach false salvation doctrine (this includes atheism, for example, which is a form of faith, except faith here is faith in the absence of deity, as well as the non-existence of "salvation", in the after-life sense). The beast is every government that outlaws the "saved" kind Christianity (e.g. Europe of Middle Ages, many of today's moslem countries, North Korea, Soviet Russia, etc.). And the dragon is just the devil. The collective false prophet looks like the lamb because he offers his own doctrines as true, just as Christ would offer His, causing people to worship his religion/faith/"image"/idol, just as Christ tried to get people to worship truth/God. He would share ethics with the beast, and would try to get people to obey certain laws of the beast that God would just never give us faith to do, like burning incense to an idol or facing death (Rome the first few centuries A.D.), for example. Often the false prophet would be in charge of the beast altogether. For example in ancient Rome the caesar would also be the high priest. Another example is king Nebuchadnezzar of ancient Babylon erecting for himself a statue and making a decree to cause everyone worship his image. A recent example is Hitler when he outlawed sheltering the Jews.

Please note, that by "saved" Christianity I do not mean Catholic church, because they also believe they'll be saved, but this faith is based on the doctrine of Purgatory, which says everyone will be saved eventually. The kind I meant is the one that preaches that we shall be saved as long as we keep our souls washed of sins (even the unforgivable can be forgiven, since God's mercy triumphs over all of His condemnations, James 2:13), and clothed with the robe of the abundance of the Holy Spirit (lukewarm heart does not count, Rev. 3:16). Hope this helps.