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.
