Trope - Fanlore

(n.) A common theme, seen repeatedly in various works.

Here is a definition from tvtropes.org:

Tropes are devices and conventions that a writer can reasonably rely on as being present in the audience members' minds and expectations. On the whole, tropes are not clichés. The word clichéd means "stereotyped and trite." In other words, dull and uninteresting. We are not looking for dull and uninteresting entries. We are here to recognize tropes and play with them, not to make fun of them.

In fandom, the word trope is often used to describe common plot devices, such as Aliens Made Them Do It, Fuck or Die, sex pollen, etc.