Revelation 13:17 (the last chapter of the Bible) states:
"And he causes all, both small and great, rich and poor, free and slave, to receive a mark on their right hand or on their foreheads, and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name. here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666."
In Revelation 14:9-12, the third angel warns of wrath and retribution to all those who bear this mark on their heads or hands.
The number 666 is source of controversy. There are various theories about the meaning of this verse beyond Christian eschatology which typically hold that the Mark of the Beast is one way in which the antichrist will exercise power over the earth, during the period known as the Tribulation in the book of Revelation.
Opinions as to what form it will take are varied. The literal interpretation is that it will be the number 666 imprinted on the skin, but conspiracy theorists imagine all sorts of devious possibilities tied in with recent technological developments.
The number 666 retains a peculiar significance in the Culture and Psychology of Western societies. Just as many people try to avoid the "unlucky" number 13 (sometimes going to the extent of having a floor "12A" in multi-story buildings), so people find ways to avoid the "Devil's number", even in contexts that would seem on first sight to be far removed from superstition.
For example, when the giant CPU manufacturer Intel introduced the 666 MHz Pentium III in 1999, they chose to market it as the Pentium III 667 on the pretext that, since the actual clock speed was 666.666 MHz, 667 was the more accurate approximation, conveniently ignoring their own usual rounding practice: as examples, consider the earlier 66.666 MHz 486-66, the 466.666 MHz Celeron 466, or the later 866.666 MHz Pentium III 866.
In another instance, United States Highway 666 was renumbered in 2003 after controversy over the supposed reference to the Biblical beast.