Dōjin music (同人音楽 doujin ongaku?), also called otokei dōjin (音系同人?) in Japan, is a sub-category of dōjin activity. Dōjin are basically non-official self-published Japanese works which can be based on official products or completely original creations. Such products are sold online on specialized sites, on the author's own sites and in conventions such as the very popular Comic Markets.
Dōjin music isn't a musical genre in itself but is indicative of a particular means of publication. (Similarly the term "indie" in the United States is used in this way.)
Dōjin music consists very often of video game music fan arrangements. Much original Dōjin music also exists, and has been created both for dōjin games and independently, spanning many musical genres such as pop, rock, techno, trance and many more.
By nature, dōjin music is often self-produced, at low cost, by independent artists. Home-studio software is typically advantageous to dōjin music composers, as it is cheaper than studio mastering live instruments. As such, most Dōjin music has a distinct synthetic quality to it. It is common to have one live instrument (such as a guitar) backed up by synthetic orchestrations, though full instrumentation is becoming more and more common in dōjin music, such as orchestral works or dōjin jazz.