Fornication is generally consensual sexual intercourse between two people not married to each other. When one or more of the partners having consensual sexual intercourse is married to a third person, it is called adultery. Nonetheless, John Calvin viewed adultery to be any sexual act that is outside the divine model for sexual intercourse, which includes fornication.
For many people, the term carries an overtone of moral or religious disapproval, but the significance of sexual acts to which the term is applied varies between religions, societies and cultures. In modern usage, the term is often replaced with more judgment-neutral terms like premarital sex or extramarital sex.