A lesbian is a homosexual woman. Lesbians are sexually and romantically attracted to other women. It could be argued that such a person is not a lesbian (as a noun) but lesbian (as an adjective). This would depend on self identification, and varies among lesbians/lesbian women.
The name comes from the island of Lesbos, birthplace of Sappho, a Greek poetess who may have been lesbian. The adjective Sapphic can be used instead of lesbian.