In general, any person, idea, or organization opposed to Christ and His Church; in particular, a signal enemy of Christ who is to appear before the Last Judgment and seduce many before his destruction by Christ.
Daniel, 7, speaks of him as the "man of sin."
Some see allusions to him in the "False Christs," the "abomination of desolation," and "the one who shall come in his own name" of the Gospel.
Saint Paul in 2 Thessalonians, 2, quite fully describes him and his destiny.
In his first two epistles Saint John speaks of several Antichrists, but more especially of one par excellence.
New Catholic Dictionary