In Rev 13:16-18, the "mark of the beast" was mentioned - the only time the bible actually refers to it. The word "mark" here in the Greek language means a tattoo or an etching or something that's scratched onto something. It says it's going to be upon the right hand or the forehead, and that you can't buy or sell unless you have the mark of the beast.
The mark of the beast is 666 (according to the book of Revelations). Some has taken this number to be one short of "777" - whereby the number 7 represents God's perfection and shows that Antichrist is short of that perfection.
Yet, Dr. Mark Hitchcock feels that this number is more likely to be the numerical value of his name. In the Greek and Hebrew language, each alphabet has a numerical value. Thus when we add up the numbers that coincide with the letters in his name, it will total up to 666.