A guest comic (or guest strip) is an issue of a comic strip that is created by a different person (or people) than usual.
Guest comics are usually requested of other artists by the usual creator of a webcomic.
This may be done for a variety of reasons: Guest comics are usually not meant to be canonical with the normal comic; their purpose is primarily entertainment.
Webcomic creators that are in the same webcomic syndicate (e.g. Dayfree Press) frequently do guest comics for each other, sometimes simultaneously.
This article related to webcomics is a stub.