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 gay 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.