A lemon is usually a fanatic that contains explicit sexual intercourse. I think it is both used for heterosexual fanatics as for homosexual ones. Usually lemons are also marked as nc-17, or M (mature).
What Does 'Lemons' Mean In Fanfiction?
A lemon in fanfiction is a fic with the characters participating in sexual activities. In most of these kinds of fics, the characters are very out of character.