What is 5th Season Lesbian?
The gratuitous introduction of a lesbian character, characters,relationship or situation in order to boost flagging interest in a television, or comic series. Usually happens in the fourth or fifth season of the series lifespan. The intent is more to spark interest with provocative girl on girl action than to create diversity.
The classic example would be Willows outing in the last few seasons of Buffy the Vampire Slayer. Willow and Tara would be the 5th season lesbians.