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 also used to refer to fictional characters who are dressed as other characters from a different anime/manga/game/etc.
Photos should only be tagged with the character being cosplayed and photo. In the case of anime characters cosplaying other characters, add a xxx_(cosplay) tag. For example, if you have a drawing of Remilia cosplaying as Reimu, you should use the following tags:
Note: Besides adding the cosplay tags, ensure to also tag the associated copyrights that go along with the character being cosplayed.