Lesbian is a sexual orientation. A lesbian is someone who is sexually attracted to someone of the same gender. Women, trans and non binary people can identify as being lesbian.
Sources:
https://www.acog.org/womens-health/faqs/lgbtq-teens
https://www.webmd.com/sex/what-is-lesbianism
https://www.betterhealth.vic.gov.au/health/healthyliving/lesbian-sexuality