Homosexuality is romantic or sexual attraction or behavior between members of the same sex or gender. As a sexual orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affectional, or romantic attractions" primarily or exclusively to people of the same sex; "it also refers to an individual's sense of personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them."
Homosexuality is one of the three main categories of sexual orientation, along with bisexuality and heterosexuality, within the heterosexual-homosexual continuum (with asexuality sometimes considered the fourth). Scientific and medical understanding is that sexual orientation is not a choice, but rather a complex interplay of biology and environment. Although some religious sects hold the view that homosexual activity is sinful or dysfunctional behaviour, research and studies show that homosexuality is a normal and positive variant of human sexuality. Though homosexuality is not in itself a source of negative psychological effects, prejudice and discrimination against homosexual and bisexual people has been shown to cause psychological harm.
The most common terms for homosexual people are lesbian for women and gay for men, though gay is also used to refer generally to homosexual men and women. The number of people who identify as gay or lesbian—and the proportion of people who have same-sex sexual experiences—are difficult for researchers to estimate reliably for a variety of reasons. In the modern West, according to major studies, 2% to 13% of the population is homosexual or has had some form of same-sex sexual contact within his or her lifetime. A 2006 study suggested that 20% of the population anonymously reported some homosexual feelings, although fewer participants in the study identified themselves as homosexual. Homosexual behavior is also widely observed in animals.
Many gay and lesbian people are in committed same-sex relationships, though only recently have census forms and political conditions facilitated their visibility and enumeration. These relationships are equivalent to heterosexual relationships in essential psychological respects. Homosexual relationships and acts have been admired, as well as condemned, throughout recorded history, depending on the form they took and the culture in which they occurred. Since the end of the 19th century, there has been a movement towards increased visibility, recognition and legal rights for homosexual people, including the rights to marriage and civil unions, adoption and parenting, employment, military service, and equal access to health care.