Cosplay(short for "costume play") refers to real people dressing up as fictional characters. This is somewhat similar to Halloween costumes (and they can certainly overlap), but a good cosplay outfit is often far more elaborate.
This tag is largly irrelevant for Cosbooru purposes as nearly every post will inherently be a cosplay. The default is to assume a cosplay, posts that aren't cosplays of a specific character should be tagged original.
This tag should only be used when a person is cosplaying a character who is then cosplaying as another character. In this case, add a xxx_(cosplay) tag. For example:
This post contains a Konata cosplayer dressed as Haruhi, and as such has the following tags:
