Lesbian is the term for a gay woman. The terms gay and lesbian are often separated due to the differences between the lesbian community and the gay community.
There has recently been discussion of changing the lesbian, as the current pink one, was originally the lipstick lesbian flag, with the lipstick mark removed. Not only does this isolate butch lesbians, it also means that the stripes have no meaning associated with them, like most other flags do.
One of the more popular alternate design ideas is from Tumblr user Sadlesbeandisaster. According the her stripe associations are as follows: dark orange for how lesbians don't conform to tradition gender standards, medium orange for independence from men, light orange for community, white for unique relations to womanhood, light pink serenity and peace, medium pink for love and sex, and dark pink for femininity.
Another popular flag design is by Tumblr user Apersnicketylemon. The color associations are as follows: purple for non-binary and trans lesbians, also to represent how violets were historically given between women to represent their love; pink for lipstick and femme lesbians, and to represent the beauty of feminine love; grey is for a-spec lesbians, and to represent the difficulty to navigating a 'grey area' of society. Blue represents butch lesbians, and how lesbians don't comfort to typical gender roles.
The word lesbian comes from the name of the Greek island Lesbos; the birthplace of the poet Sappho. It can't be know if Sappho was a lesbian in the modern sense of the word, but she is known for writing poems about her attraction towards women. Sappho is also the source of the word sapphic.