Lesbianism is not a woman. It is the name for women being sexually attracted to women — the orientation, the relationships, or the fact of that love. A lesbian is the person; lesbianism is the -ism, the idea. You will see it in older books, in formal writing, and in news more than in friendly talk. Many people today just say someone is a lesbian, or talk about being gay, and do not use this word at all. It is built in a simple way: lesbian plus -ism, the ending English uses to turn a person or adjective into an idea. The rest of the history is the same as lesbian: Latin Lesbiana from the island of Lesbos, Sappho’s poems about women, and a likely push from German medical language in the 1800s. A dictionary line that makes this word sound like it means “a woman” has mixed it up with lesbian.