Strictly speaking, non-mainstream music can be divided into broad sense and narrow sense.

in a broad sense, non-mainstream music is a music form corresponding to mainstream music, and some marginal music forms are also considered non-mainstream. For example, in China, pop music is mainstream music, while folk songs and campus folk songs should not be regarded as mainstream music, so they are classified as non-mainstream music in a broad sense.

what we usually talk about is non-mainstream music in a narrow sense, which mainly refers to music forms that are not accepted or understood by mainstream groups or most groups in society. Usually, it is the so-called alternative music. This kind of music has fewer forms and different styles, and it is not easy to spread widely. In a narrow sense, in addition to pop music, folk songs in China are also considered mainstream, while non-mainstream refers to some heavy metal rock and psychedelic music. In Europe and America, the mainstream is rock, punk, country, jazz, blues or rap.