Homosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the same sex or gender. It also denotes identity based on attraction, related behavior, and community affiliation.
Along with bisexuality and heterosexuality, homosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum. Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biological theories. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.
A major hypothesis involves the prenatal environment, in particular the organizational effects of hormones on the fetal brain. There is no conclusive evidence which confirms that parenting or early childhood experiences play a significant role in developing a sexual orientation. 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. Major mental health organizations overwhelmingly reject conversion therapy as ineffective, scientifically unsupported and potentially harmful.
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. The number of people who are gay or lesbian is difficult for researchers to estimate reliably, as many gay and lesbian people do not openly identify as such due to discrimination or prejudice such as heterosexism or homophobia. Homosexual behavior has also been documented in many non-human animal species, though domestic sheep are the only conclusively documented example of nonhuman animals exhibiting exclusive same-sex orientation.
Many gay and lesbian people are in committed same-sex relationships. 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 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 teenagers 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.
The word homosexual is a Greek and Latin hybrid, with the first element derived from Greek ὁμός homos, 'same' (not related to the Latin homo, 'man', as present in the genus Homo, which includes Homo sapiens and now extinct species), thus connoting sexual acts and affections between members of the same sex, including lesbianism. The first known appearance of homosexual in print is found in an 1868 letter to Karl Heinrich Ulrichs by the Austrian-born novelist Karl-Maria Kertbeny arguing against a Prussian anti-sodomy law. In 1886, the psychiatrist Richard von Krafft-Ebing used the terms homosexual and heterosexual in his book Psychopathia Sexualis. Krafft-Ebing's book was so popular among both laymen and doctors that the terms heterosexual and homosexual became the most widely accepted terms for sexual orientation.
Many modern style guides in the U.S. recommend against using homosexual as a noun, instead using gay man or lesbian. Similarly, some recommend completely avoiding usage of homosexual as it has a negative, clinical history and because the word only refers to one's sexual behavior (as opposed to romantic feelings) and thus it has a negative connotation. Gay and lesbian are the most common alternatives. The first letters are frequently combined to create the initialism LGBT (sometimes written as GLBT), in which B and T refer to bisexual and transgender people.
Gay especially refers to male homosexuality, but may be used in a broader sense to refer to all LGBTQ people. In the context of sexuality, lesbian refers only to female homosexuality. The word lesbian is derived from the name of the Greek island Lesbos, where the poet Sappho wrote largely about her emotional relationships with young women.
Although early writers also used the adjective homosexual to refer to any single-sex context (such as an all-girls school), today the term is usually used in reference to sexual attraction, activity, and orientation. The term homosocial is now used to describe single-sex contexts that are not specifically sexual. There is also a word referring to same-sex love, homophilia.
Some synonyms for same-sex attraction or sexual activity include men who have sex with men or MSM (used in the medical community when specifically discussing sexual activity) and homoerotic (referring to works of art). Pejorative terms in English include queer, faggot, fairy, poof, poofter and homo. Beginning in the 1990s, some of these have been reclaimed as positive words by gay men and lesbians, as in the usage of queer studies, queer theory, and even the popular American television program Queer Eye for the Straight Guy. The word homo occurs in many other languages without the pejorative connotations it has in English. As with ethnic slurs and racial slurs, the use of these terms can still be considered offensive. The range of acceptable use for these terms depends on the context and speaker. Conversely, gay, a word originally embraced by homosexual men and women as a positive, affirmative term (as in gay liberation and gay rights), came into widespread pejorative use among young people in the early 2000s.
Same-sex relations are documented throughout history and varied in acceptance depending on time and place. However, many historians and anthropologists warn against using terms like "homosexuality" to describe these practices, because that may draw false equivalencies between acts that, while technically similar, may have been viewed as completely different acts. In many cultures, same-sex relations were not always considered sexual in the same way that "reproductive" sex between men and women was—particularly lesbian sex. Modern-style homosexual partnerships, including marriages, are even more uncommon, probably because marriages have almost always been oriented around producing children. Other scholars argue that there are significant continuities between historical and modern homosexuality.
In cultures influenced by Abrahamic religions, the law and the church established sodomy as a transgression against divine law or a crime against nature. The condemnation of anal sex between males, however, predates Christian belief. Throughout the majority of Christian history, most Christian theologians and denominations have considered homosexual behavior as immoral or sinful.
Many historical figures, including Socrates, Lord Byron, Edward II, and Hadrian, have had terms such as gay or bisexual applied to them. Some scholars have regarded uses of such modern terms on people from the past as an anachronistic introduction of a contemporary construction of sexuality that would have been foreign to their times. Other scholars see continuity instead.
In social science, there has been a dispute between "essentialist" and "constructionist" views of homosexuality. The debate divides those who believe that terms such as "gay" and "straight" refer to objective, culturally invariant properties of persons from those who believe that the experiences they name are artifacts of unique cultural and social processes. "Essentialists" typically believe that sexual preferences are determined by biological forces, while "constructionists" assume that sexual desires are learned. The philosopher of science Michael Ruse has stated that the social constructionist approach, which is influenced by Foucault, is based on a selective reading of the historical record that confuses the existence of homosexual people with the way in which they are labelled or treated.