Sexual intercourse, or coitus, refers in a strict biological sense to the insertion of the male's penis into the female's vagina for the purpose of reproduction. Sexual intercourse is found among all mammalian species.

Intercourse has traditionally been viewed as the natural endpoint of all sexual contact between a man and a woman. However, the meaning of the term has been broadened in recent years to include a wider range of behaviors and a wider set of motivations and intentions.

In both popular and professional usage, intercourse now labels at least three different sex acts, two of which are not directly tied to conceiving a child. These three types of intercourse are: vaginal intercourse, involving vaginal penetration by the penis, possibly to the point of male ejaculation and female orgasm; oral intercourse, involving oral caress of the sex organs (male or female), possibly to the point of orgasm; and anal intercourse, involving insertion of the male's penis into his partner's anus. The latter two of these behaviors may be the endpoints of a sexual encounter or they may be acts of foreplay leading to each other or to vaginal intercourse.

Moreover, intercourse is not limited to partnerships between individuals of opposite genders. Same-sex or homosexual encounters, involving oral or anal penetration or stimulation, are also referred to as sexual intercourse. Some also include digital (use of fingers or hands) intercourse or mutual masturbation as yet another form of intercourse.