Original soundtrack

A soundtrack, also written sound track, can be recorded music accompanying and synchronized to the images of a motion picture, book, television program or video game; a commercially released soundtrack album of music as featured in the soundtrack of a film or TV show; or the physical area of a film that contains the synchronized recorded sound.

In movie industry terminology usage, a sound track is an audio recording created or used in film production or post-production. Initially the dialogue, sound effects, and music in a film each has its own separate track (dialogue track, sound effects track, and music track), and these are mixed together to make what is called the composite track, which is heard in the film. A dubbing track is often later created when films are dubbed into another language. This is also known as a M & E track (music and effects) containing all sound elements minus dialogue which is then supplied by the foreign distributor in the native language of its territory.

The contraction soundtrack came into public consciousness with the advent of so-called "soundtrack albums" in the late 1940s. First conceived by movie companies as a promotional gimmick for new films, these commercially available recordings were labeled and advertised as "music from the original motion picture soundtrack", or "music from and inspired by the motion picture." These phrases were soon shortened to just "original motion picture soundtrack." More accurately, such recordings are made from a film's music track, because they usually consist of the isolated music from a film, not the composite (sound) track with dialogue and sound effects.

The abbreviation OST is often used to describe the musical soundtrack on a recorded medium, such as CD, and it stands for Original Soundtrack; however, it is sometimes also used to differentiate the original music heard and recorded versus a rerecording or cover of the music.

...
Wikipedia