Gay is romantic or sexual attraction or behavior among members of the same sex. As a sexual orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affection, or romantic attractions primarily 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. The longstanding consensus of the behavioral and social sciences and the health and mental health professions is that homosexuality per se is an example of normal and positive variation in human sexual orientation. Currently the most common adjectives in use are lesbian for women and gay for men, though gay can refer to either men or 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, major studies indicate a prevalence of 2% to 13% of the population.

A 2006 study suggested that 20% of the population anonymously reported some homosexual feelings, although relatively few participants in the study identified themselves as homosexual. Homosexual behavior in animals is also widely encountered.

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 1800s, 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.

The use of gay to mean "homosexual" was in origin merely an extension of the word's sexualized connotation of "carefree and uninhibited", which implied a willingness to disregard conventional or respectable sexual mores. Such usage is documented as early as the 1920s, and there is evidence for it before the 20th century, although it was initially more commonly used to imply heterosexually unconstrained lifestyles, as in the once-common phrase "gay Lothario",or in the title of the book and film The Gay Falcon (1941), which concerns a womanizing detective whose first name is "Gay."

Well into the mid 20th century a middle-aged bachelor could be described as "gay", indicating that he was unattached and therefore free, without any implication of homosexuality. This usage could apply to women too. The British comic strip Jane was first published in the 1930s and described the adventures of Jane Gay. Far from implying homosexuality, it referred to her free-wheeling lifestyle with plenty of boyfriends (while also punning on Lady Jane Grey)