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 implicates the prenatal environment, specifically the organizational effects of hormones on the fetal brain. There is no substantive evidence which suggests parenting or early childhood experiences play a 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 sexual orientation change efforts (such as conversion therapy) as ineffective, scientifically unsupported, potentially harmful, and rooted in stigma rather than evidence.

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.^{[citation needed]} 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 used exclusively 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.^{[citation needed]}