Most literally, a 'track' is a section of a medium (tape, wax, vinyl) on which a physical (or magnetic) track has been created due to a recording being made or pressed there. Even during times when audio usually did correspond to a physical track, when people said 'track' they were often talking about the song/piece of audio, rather than the physical track.
Now that many media (e.g. mp3 files) don't have a physical track, the word 'track' seems to mean a recording of the type that would have been a track on a record (or other medium). So a two second mp3 file of someone laughing isn't a 'track' unless it has actually come from a sequence of other sound files arranged like an album; A song can usually be called a 'track', because we are used to songs being tracks on albums.