Source : Webster's Revised Unabridged Dictionary (1913)

Antichrist \An"ti*christ\, n. [L. Antichristus, Gr. ?; ? against
   + ?.]
   A denier or opponent of Christ. Specif.: A great antagonist,
   person or power, expected to precede Christ's second coming.

Source : WordNet®

Antichrist
     n : (Christianity) the adversary of Christ (or Christianity)
         mentioned in the New Testament; the Antichrist will rule
         the world until overthrown by the Second Coming of Christ