Homosexuality is the romantic or sexual attraction or behavior among members of the same sex, situationally or as an enduring disposition. As a sexual orientation, homosexuality is considered to lie within the heterosexual-homosexual continuum of human sexuality, and refers to an individual's identity based on those attractions and membership in a community of others who share them.

Homosexuality is widely encountered in the animal kingdom. Among humans, the prevalence of homosexuality is difficult to determine accurately; studies suggest between two and twenty percent of the population exhibit some degree of homosexual sensibility, though in many earlier cultures homosexual relations were prevalent. Throughout history, individual aspects of homosexuality have been admired or condemned according to various societies' sexual norms. When praised, those aspects were seen as a way to improve society; when condemned, particular activities were seen as a sin or a disease, and some homosexual behavior was prohibited by law. Since the middle of the 20th century homosexuality has been gradually delisted as a disease and decriminalized in nearly all developed countries. However, the legal status of homosexual relations varies widely by country and there remain jurisdictions in which certain homosexual behaviors are crimes with severe penalties including death.

Many homosexual people hide their feelings and activities out of fear of disapproval or aggression; they are commonly said to be closeted. Disclosing one's homosexual or bisexual orientation is known as coming out (of the closet). Efforts toward emancipation of homosexuality as it is currently understood began in the 1860s; since the mid-1950s there has been an accelerating trend towards increased visibility, acceptance, and civil rights for lesbian, gay and bisexual people. Nevertheless, heterosexism and homophobia persist, and in particular young people subjected to it are at greater risk of socialization difficulties including suicide. Currently the most common adjectives in use are lesbian for women and gay for men, though some prefer other terms or none at all.