Homosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the same sex or gender.[1][2][3] As a sexual orientation, homosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" exclusively to people of the same sex or gender.[4] It "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions."[5][6]

Along with bisexuality and heterosexuality, homosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum.[5] Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biological theories.[7][8][9] There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.[7][10][11] A major hypothesis implicates the prenatal environment, specifically the organizational effects of hormones on the fetal brain.[7][9] There is no substantive evidence which suggests parenting or early childhood experiences play a role in developing a sexual orientation.[7][12] Scientific research shows that homosexuality is a natural and normal variation in human sexuality and is not in and of itself a source of negative psychological effects.[5][13] There is insufficient evidence to support the use of psychological interventions to change sexual orientation.[14][15]

The most common terms for homosexual people are lesbian for females and gay for males, but the term gay also commonly refers to both homosexual females and males. Other common labels that include gay people are LGBTQ and queer. The percentage of people who are gay or lesbian and the proportion of people who are in same-sex romantic relationships or have had same-sex sexual experiences are difficult for researchers to estimate reliably for a variety of reasons, including many gay and lesbian people not openly identifying as such due to prejudice or discrimination such as homophobia and heterosexism.[16] Homosexual behavior has also been documented in many non-human animal species,[22] though humans are one of only two species known to exhibit a homosexual orientation (the other is sheep).[7]

Many gay and lesbian people are in committed same-sex relationships. These relationships are equivalent to heterosexual relationships in essential psychological respects.[6] 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.[23] Since the end of the 20th century, there has been a global movement towards freedom and equality for gay people, including the introduction of anti-bullying legislation to protect gay children at school, legislation ensuring non-discrimination, equal ability to serve in the military, equal access to health care, equal ability to adopt and parent, and the establishment of marriage equality.