An insert song is a vocal song played during an episode's scenes rather than in its opening or ending sequences: over a performance, a montage, a battle, or a key emotional beat. It is distinct from the background score and from the OP and ED.
The insert song is the soundtrack's scalpel. A series' composer scores most scenes instrumentally, but certain moments get a vocal track laid over the action: a performance inside the story, a montage, a battle the staff want to elevate, a farewell. Because vocals claim the whole sound field, directors spend them sparingly, and scarcity is what makes them land.
The category includes some of the medium's most famous sequences. Concert scenes in music series are built around insert songs; idol and band anime are essentially delivery systems for them. In action series a single insert song, returning at the right moment, can become a franchise's signature as surely as any character.
For identification purposes the distinctions matter. An insert song is not the OP, the ED, or background score (the BGM), and it is credited separately on soundtrack releases. Fans who can name which insert song plays in which scene hold a specific and testable kind of series knowledge. Soundtrack albums collect insert songs as headline tracks, and a series' music sales are often carried by one insert song that outlives the season it was written for.
In use"The ballad that starts under the final confrontation, performed by the series' singer, is an insert song."