Fornication generally refers to consensual sexual intercourse between two people who are not married to each other. When a married person has consensual sexual relations with one or more partners whom they are not married to, it is called adultery. John Calvin viewed adultery to be a sexual act that is considered outside of the divine model for sexual intercourse between married individuals, 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, extramarital sex, or recreational sex.