Spamfic

Quoting from the Ranma 1/2 Fan Fiction Wiki:

Spam-fics, originally known as Silly-fics and now generally referred to as Omake, are generally short stories designed to provoke a response. In many cases they are a way that a writer can explore a story concept or character pairing without any expectations on the part of the reader.

Like the omake that's produced for anime series, spamfics range from providing foreshadowing to the main plot of their parent works, to giving secondary characters A Day in the Limelight that might or might not be canonical, or simply be a bit of foolishness that's related to the main work In Name Only.

Spamfics can also explore ideas that can't support a full-length story. They're generally short works.

Contrast with Feghoot (which is always comedic) or Drabble (which is always short).