coupling track; B-side
Refers to the additional song(s) included with a single release, alongside the main A-side track. Common in J-pop and music industry contexts.
The coupling track on this single is also a really good song.
The B-side was unexpectedly popular and was later included on the album as well.
From English 'coupling' + 曲 (song). The term 'coupling track' is a wasei-eigo usage referring to the additional songs paired with a single's main track.