Outsider music is songs and compositions by musicians who are not part of the commercial music industry. They write songs that ignore standard musical or lyrical conventions, either because they have no formal training or because they disagree with formal rules. This type of music, which is often bizarre and emotionally stark, has few outlets; performers or recordings are often promoted by word of mouth or through fan chat sites, usually among communities of music collectors and music connoisseurs. Outsider musicians usually have much "greater individual control over the final creative" product either because of a low budget or because of their "inability or unwillingness to cooperate" with modifications by a record label or producer.

While a small number of outsider musicians became notable because they have few musical skills, the majority of outsider artists are appreciated for their unique, uncompromising, and creative music.

Irwin Chusid, who wrote an influential book on Outsider music called "Songs in the Key of Z", makes a distinction between truly Outsider music and music that is merely "out there". The latter is made by musicians who are consciously pushing boundaries as part of an avant-garde tradition, while Outsider music is made by musicians with a lack of awareness that they are operating outside musical conventions. In some cases, such naivety stems from a mental illness or a reclusive lifestyle that may influence their musical style. Sometimes the boundaries between "out there" and Outsider are blurry.