A lesbian is a homosexual woman or girl. The word is also used for women in relation to their sexual identity or sexual behavior, regardless of sexual orientation, or as an adjective to characterize or associate nouns with female homosexuality or same-sex attraction. The concept of "lesbian" to differentiate women with a shared sexual orientation evolved in the 20th century. Throughout history, women have not had the same freedom or independence as men to pursue homosexual relationships, but neither have they met the same harsh punishment as homosexual men in some societies. Instead, lesbian relationships have often been regarded as harmless, unless a participant attempts to assert privileges traditionally enjoyed by men. As a result, little in history was documented to give an accurate description of how female homosexuality was expressed. When early sexologists in the late 19th century began to categorize and describe homosexual behavior, hampered by a lack of knowledge about homosexuality or women's sexuality, they distinguished lesbians as women who did not adhere to female gender roles. They classified them as mentally ill—a designation which has been reversed since the late 20th century in the global scientific community.

Women in homosexual relationships in Europe and the United States responded to the discrimination and repression either by hiding their personal lives, or accepting the label of outcast and creating a subculture and identity. Following World War II, during a period of social repression when governments actively persecuted homosexuals, women developed networks to socialize with and educate each other. Gaining greater economic and social freedom allowed them to determine how they could form relationships and families. With second-wave feminism and the growth of scholarship in women's history and sexuality in the late 20th century, the definition of lesbian broadened, leading to debate about the term's use. While research by Lisa M. Diamond identified sexual desire as the core component for defining lesbians, some women who engage in same-sex sexual activity may reject not only identifying as lesbians but as bisexual as well. Other women's self-identification as lesbian may not align with their sexual orientation or sexual behavior. Sexual identity is not necessarily the same as one's sexual orientation or sexual behavior, due to various reasons, such as the fear of identifying their sexual orientation in a homophobic setting.

Portrayals of lesbians in the media suggest that society at large has been simultaneously intrigued and threatened by women who challenge feminine gender roles, as well as fascinated and appalled with women who are romantically involved with other women. Women who adopt a lesbian identity share experiences that form an outlook similar to an ethnic identity: as homosexuals, they are unified by the heterosexist discrimination and potential rejection they face from their families, friends, and others as a result of homophobia. As women, they face concerns separate from men. Lesbians may encounter distinct physical or mental health concerns arising from discrimination, prejudice, and minority stress. Political conditions and social attitudes also affect the formation of lesbian relationships and families in the open.