One-shot

From UnknowableWiki

A one-shot is a fanfiction written with no intentions of continuation. It consists of a single chapter, usually narrating a short, concise plot or a missing scene from the books.

Things to Avoid

Because one-shots are relatively short pieces, you should not spend too much time giving every nuance of each character's personalities. Obviously, it is important to make each character your own, but it is permissible to leave out details that the reader is already (or should already be) aware of and spend more time focusing on the story. However, you should usually mention the setting either at the beginning or in the summary, since short fanfics usually don't give much opportunity for the reader to figure out the setting.