Coitus: Sexual intercourse (or coitus or copulation) is principally the insertion and thrusting of the penis, usually when erect, into the vagina for sexual pleasure, reproduction, or both. This is also known as vaginal intercourse or vaginal sex. Other forms of penetrative sexual intercourse include anal sex (penetration of the anus by the penis), oral sex (penetration of the mouth by the penis or oral penetration of the female genitalia), fingering (sexual penetration by the fingers), and penetration by use of a dildo (especially a strap-on dildo). These activities involve physical intimacy between two or more individuals and are usually used among humans solely for physical or emotional pleasure and can contribute to human bonding.

There are different views on what constitutes sexual intercourse or other sexual activity, which can impact views on sexual health. Although the term sexual intercourse, particularly the variant coitus, generally denotes penile–vaginal penetration and the possibility of creating offspring, it also commonly denotes penetrative oral sex and penile–anal sex, especially the latter. It is usually defined by sexual penetration, while non-penetrative sex (such as mutual masturbation and non-penetrative forms of cunnilingus) has been termed outercourse, but non-penetrative sex may also be considered sexual intercourse. The term sex, often a shorthand for sexual intercourse, can mean any form of sexual activity. Because people can be at risk of contracting sexually transmitted infections during these activities, safe sex practices are advised, although transmission risk is significantly reduced during non-penetrative sex.

Various jurisdictions have placed restrictive laws against certain sexual acts, such as incest, sexual activity with minors, prostitution, rape, zoophilia, sodomy, premarital and extramarital sex. Religious beliefs also play a role in personal decisions about sexual intercourse or other sexual activity, such as decisions about virginity, or legal and public policy matters. Religious views on sexuality vary significantly between different religions and sects of the same religion, though there are common themes, such as prohibition of adultery.

Reproductive sexual intercourse between non-human animals is more often termed copulation, and sperm may be introduced into the female's reproductive tract in non-vaginal ways among the animals, such as by cloacal copulation. For most non-human mammals, mating and copulation occur 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 regardless of whether or not the female is in estrus, and to engage in sex acts with same-sex partners. Like humans engaging in sexual activity primarily for pleasure, this behavior in the aforementioned animals is also presumed to be for pleasure, and a contributing factor to strengthening their social bonds.