antichrist

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The great enemy of Christ expected by the early Church and historically by many branches of Christianity to rise to power in the last days before the Second Coming.
• noun One who actively denies or opposes Christianity.
• noun A false Christ.

from The Century Dictionary.

• noun An opponent of Christ; a person or power antagonistic to Christ.
• noun The word occurs in the Scriptures only in the Epistles of John; but the same person or power is elsewhere referred to (2 Thes. ii. 1–12; 1 Tim. iv. 1–3; 2 Pet. ii. 1). Interpreters of Scripture differ in their understanding of these references. Some suppose them to relate to a lawless but impersonal power, a spirit opposed to Christianity; some to a historic personage or potentate, as Caligula, Titus, the pope, or Luther; some to a great power for evil yet to be manifested and gathered about a central personal agency. Roman Catholic writers commonly interpret the word generically of any adversary of Christ and of the authority of the church, but specifically as the last and greatest persecutor of the Christian church at the end of the world. The name has also been applied to the pretenders to the messiahship, or false Christs (Mat. xxiv. 24), who have arisen at various periods, as being antagonistic to the true Christ. Of these as many as sixty-four have been reckoned, including some of little importance, and also some, as Mohammed, who cannot properly be classed among them.

from the GNU version of the Collaborative International Dictionary of English.

• noun A denier or opponent of Christ. Specif.: A great antagonist, person or power, expected to precede Christ's second coming.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who works against the teachings of Christ.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (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