Что (кто) такое B-side - определение

The A-side and B-side are the two sides of phonograph records and cassettes; these terms have often been printed on the labels of two-sided music recordings. The A-side usually features a recording that its artist, producer, or record company intends to be the initial focus of promotional efforts and radio airplay and hopefully become a hit record. The B-side (or "flip-side") is a secondary recording that typically receives less attention, although some B-sides have been as successful as, or more so than, their A-sides.

Use of this language has largely declined in the 21st century as the music industry has transitioned away from analog recordings towards digital formats without physical sides, such as CDs, downloads and streaming. Nevertheless, some artists and labels continue to employ the terms A-side and B-side metaphorically to describe the type of content a particular release features, with B-side sometimes representing a "bonus" track or other material. The term B-side carries a more expansive definition in the K-pop industry, referring to all tracks on an album that are not marketed as title tracks.