That's easy (not the 666 part, but the definition of the antichrist). The antichrist is a group of people, those who deny that Jesus is the Christ, or Messiah, who deny that there is both a a Father(YHWH) and a Son(Jesus Christ), those who deny that Christ came in the flesh. There is no ambiguity as to that definition:
1 John 2:18
Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour.
(Whole Chapter: 1 John 2 In context: 1 John 2:17-19)
1 John 2:22
Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist?he denies the Father and the Son.
(Whole Chapter: 1 John 2 In context: 1 John 2:21-23)
1 John 4:3
but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
(Whole Chapter: 1 John 4 In context: 1 John 4:2-4)
2 John 1:7
Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist.
(Whole Chapter: 2 John 1 In context: 2 John 1:6-8)
As for the ?number of the Beast?, 666, this remains a mystery in that no one can absolutely prove the meaning of the numbers to date.