A lesbian is a female homosexual: a female who experiences romantic love or sexual attraction to other females. The term lesbian is also used to express 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, is a 20th-century construct. Throughout history, women have not had the same freedom or independence to pursue homosexual relationships as men, but neither have they met the same harsh punishment as homosexual men in some societies. Instead, lesbian relationships have often been regarded as harmless and incomparable to heterosexual ones unless the participants attempted to assert privileges traditionally enjoyed by men. As a result, little in history was documented to give an accurate description of how female homosexuality is 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 and incorrectly designated them mentally ill—a designation which has been reversed in the global scientific community.