Sexual intercourse, in its biological sense, is the act in which the male reproductive organ (in humans and other higher animals) enters the female reproductive tract, called copulation or coitus in other reference. The two entities may be of opposite sexes, or they may be hermaphroditic, as is the case with snails.

Traditionally, intercourse has been viewed as the natural endpoint of all sexual contact between a man and a woman, and is commonly confined to this definition today. The meaning of the term, however, has been broadened in recent years, and now labels at least three different sex acts. These three types of intercourse are: vaginal intercourse, involving vaginal penetration by the penis; oral intercourse, involving oral caress of the sex organs (male or female); and anal intercourse, involving insertion of the male's penis into his partner's anus.

Sex acts that involve the use of fingers or hands or mutual masturbation are more often referred to as outercourse (with oral sex at times listed as an aspect),

For most non-human animals, sexual intercourse is used mainly for reproduction, at the point of estrus (the most fertile period of time in the female's reproductive cycle),"Females of almost all species except man will mate only during their fertile period, which is known as estrus, or heat..."

In animals

Many animals which live in the water use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates (such as reptiles, some fish, and most birds) occur via cloacal copulation (see also hemipenis), while mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.

However, some terrestrial arthropods do use external fertilization. For primitive insects, the male deposits spermatozoa on the substrate, sometimes stored within a special structure, and courtship involves inducing the female to take up the sperm package into her genital opening; there is no actual copulation. In groups such as dragonflies and many spiders, males extrude sperm into secondary copulatory structures removed from their genital opening, which are then used to inseminate the female (in dragonflies, it is a set of modified sternites on the second abdominal segment; in spiders, it is the male pedipalps). In advanced groups of insects, the male uses its aedeagus, a structure formed from the terminal segments of the abdomen, to deposit sperm directly (though sometimes in a capsule called a "spermatophore") into the female's reproductive tract.