Doujin ongaku (同人音楽), Japanese for doujin music, is a sub-category of doujin activity. Doujin works are basically non-official self-published Japanese works, which can be based on official products or completely original creations.

Doujin ongaku is not a genre, it is the way of distribution. The groups producing doujin music are called doujin circles and can be solo or band projects. Doujin ongaku consists mostly of video game music and anime soundtracks fan arrangements. It is often restricted to synthetic digital productions, which allow self-production at low cost.

Sites: Wikipedia.