Refers to the main musical piece associated with a film, TV show, game, or event. Often interchangeable with テーマ曲.
See also: テーマ曲
The theme song of this drama is very popular.
I tried playing the movie's theme music on the piano.
Compound of 主題 (theme, subject) and 曲 (musical piece). The exact origin is uncertain, but it likely arose as a direct translation of the English 'theme music' or 'theme song'.