Homosexuality is romantic or sexual attraction or behavior between members of the same sex or gender. As an orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affectionate, 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 a fourth). Scientific and medical understanding is that sexual orientation is not a choice, but rather a complex interplay of biological and environmental factors, especially with regard to early uterine environment. While there are those who still hold the view that homosexual activity is "unnatural" or "dysfunctional", research has shown that homosexuality is an example of a normal and natural variation in human sexuality and is not in and of itself a source of negative psychological effects. Prejudice and discrimination against homosexual and bisexual people, however, have been shown to cause psychological harm.
The most common terms for homosexual people are lesbian for females and gay for males, though gay is also used to refer generally to both homosexual males and females. 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. According to major studies, 2% to 10% of people have had some form of same-sex sexual contact within their lifetime. In a 2006 study, 20% of respondents anonymously reported some homosexual feelings, although only 2-3% identified themselves as homosexual. Homosexual behavior has also been documented and is observed in many non-human animal species
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 global 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, equal access to health care, and the introduction of anti-bullying legislation to protect LGBT minors.