Great to hear from you again. We assume your question is coming from 1 John 2: 18. We do believe that John is using the word "antichrist" as an adjective in this particular spot. If you read on down to verse 22 you will see John give you a definition of the word antichrist.
1Jn 2:22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.
There has been teaching that the Antichrist will rise to power during the Tribulation, but we haven't been able to find anywhere in the Bible calling him the Antichrist. We have seen the Bible talk about a False prophet, Beast, Woman wearing scarlet (riding on the Beast) but oddly no mention of him being called the Antichrist. There is two mentions of the "Son of Perdition" once in the Gospel of John, (which appears to be talking about Judas) and the other mention is in
2Th 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
2Th 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
Here it appears to be talking about what is widely believed as the antichrist. We are wanting to dig a little bit further into this. But for a quick answer antichrist seems to be a word used to describe the people that deny the Father and the Son. Although there does seem to be references to a single person that will come during the tribulation.
We hope this helps for now and we will keep you posted on anything else we find. Have a great day.