Overview
Fan art (ファンアート, fan'āto) are works of illustrations that are not published in fanzines/doujin works, but mainly on the Internet. It is attached to illustrations with a strong fan consciousness.
Fanart or fanart is a pixiv tag for fan art. Due to pixiv tags not having spaces it can be hard to write a tag in English. Some users have resorted to getting rid of the spaces and combining the whole tile together.
For more details refer to the article Fan Art.
External Links
English
Japanese
ファンアートとは [単語記事] - ニコニコ大百科 (NicoNico Encyclopedia)