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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; 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), while the term sex, in the context of sexual intimacy, is often understood more widely to include any mutual genital stimulation.
For most non-human animals, sexual intercourse occurs at the point of estrus (the most fertile period of time in the female's reproductive cycle), which increases the chances of successful impregnation. However, bonobos, dolphins, and chimpanzees are known to engage in sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners. In most instances, humans have sex primarily for pleasure. This behavior in the above mentioned animals is also presumed to be for pleasure, which in turn strengthens social bonds.
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