A guest comic is an issue of a comic strip

Comic strip

A comic strip is a sequence of drawings arranged in interrelated panels to display brief humor or form a narrative, often serialized, with text in balloons and captions....

 that is created by a different person (or people) than usual. The practice is especially common in webcomic

Webcomic

Webcomics, online comics, or Internet comics are comics published on a website. While many are published exclusively on the web, others are also published in magazines, newspapers or often in self-published books....

s.

Guest comics in webcomics

Guest comics are usually requested of other artists by the usual creator of a webcomic. This may be done for a variety of reasons:

• The creators are friends.
• The usual creator will be unable to update his or her own comic for some length of time, so they use guest comics to fill in the gap.
• The usual creator enjoys (or thinks the audience will enjoy) seeing how other artists draw their characters.
• A major phase of the plot has ended and the creator wants to take a break but keep the audience entertained.

Guest comics are usually not meant to be canonical

Canon (fiction)

In the context of a work of fiction, the term canon denotes the material accepted as "official" in a fictional universe's fan base. It is often contrasted with, or used as the basis for, works of fan fiction, which are not considered canonical...

 with the normal comic; their purpose is primarily entertainment.

Sometimes, guest comics are not published as if they were regular comics, but instead exhibited on the comic's website in a separate section; e.g. White Ninja Comics. Webcomic creators that are in the same webcomic syndicate (e.g. Dayfree Press

Dayfree Press

Dayfree Press is an online webcomic syndicate. Each Dayfree member has a special advertisement banner on their webpage that advertises the comics of another member-site...

) frequently do guest comics for each other, sometimes simultaneously.