Who or what is the "anti-christ?"
The term antichrist is mentioned only a few times in the Bible and is used in singular and plural forms, implying there will be both a particular Antichrist as well as preceding antichrists. Because of their heinous crimes, men such as Nero, Hitler, and Stalin have been termed antichrists.
In 1 John 2:18 the term implies a particular man who, before the Second Coming, will persecute the Church with a ferocity never before experienced. 1 John 2:22 tells us that anyone who opposes the Father is an antichrist. 1 John 4:3 says, "Every spirit that does not acknowledge Jesus does not belong to God. This is the spirit of the antichrist that, as you have heard, is to come, but in fact is already in the world."
Bishop Polycarp of Smyrna (d. cir. A.D. 156) said in his Letter to the Philippians, "Everyone who does not confess that Jesus Christ has come in the flesh is an antichrist."
Paul's reference to "the Lawless One" seems to be another allusion to the Antichrist. He's described as a man who will be empowered by Satan, come forth in the end times, savagely persecute the Church, proclaim himself a god, and, through the signs and wonders he performs, be worshipped by most people living at that time.
Finally, when the time comes, the Antichrist will be destroyed by Jesus Christ when he returns in glory (2 Thes 2:1-12).
By the way, the Church has made no formal definition on the matter, so Catholics are free to hold that the Antichrist was or will be an individual, a group of people, or a particularly wicked era.