noun
insert song; featured song; soundtrack song
Refers to a song inserted into a movie, TV drama, anime, or commercial, as opposed to the main theme song. Often used in media contexts.
このアニメの挿入歌はとても人気がある。
The insert songs in this anime are very popular.
映画の挿入歌として有名な曲が使われた。
A famous song was used as the movie's insert song.
BGM is background music, usually instrumental and not a featured vocal song. 挿入歌 is a distinct song, often with vocals, used at a specific moment.
Compound of 挿入 (insertion) + 歌 (song). The term is a straightforward modern coinage for media contexts.