In terms of recorded music, a bonus track is a piece of music which has been included on specific releases or reissues of an album. This is most often done as a promotional device or as an incentive to customers to purchase albums they may not otherwise purchase. In contrast to hidden tracks, bonus tracks are included on track listings and are not essentially different to the other album tracks. Methods and media for sound recording are varied and have undergone significant changes between the first time sound was actually recorded for later playback until now. ... Music is an art, entertainment, or other human activity which involves organized and audible sound, though definitions vary. ... An album is a collection of related audio tracks, released together commercially in an audio format to the public. ... In the field of recorded music, a hidden track is a piece of music which has been deliberately placed on a compact disc, audio cassette, vinyl record or other recorded medium in such a way as to avoid detection by the casual listener. ...

Many international releases - most commonly Japanese releases of European or American albums, but not confined to just Japan, or just music going from West to East - contain a few bonus tracks. These are often b-sides from singles; as the music industry in America and Europe moves much faster than elsewhere, more singles are released and so B-sides are released as Bonus Tracks so that foreign markets do not miss out. In recorded music, the terms A-side and B-side refer to the two sides of 7 inch vinyl records on which singles have been released since the 1950s. ...