Fornication is a word for sex, especially sex that takes place outside of marriage. In some religions, fornication is considered a sin.
This is not the kind of word friends use when sitting around talking about sex. This is an old-fashioned word that you can definitely spot in the Bible and other holy books, because it refers to sex that is considered wrong because it is not between a man and a wife. So in the original sense, fornication is a synonym for adultery. But people also humorously use fornication to mean any kind of sex at all.
voluntary sexual intercourse between persons not married to each other
extramarital sex that willfully and maliciously interferes with marriage relations