(In the context of Compact Discs) A track is a song, or a section of the whole CD.
(In the context of a 4-track) A track is a channel or a layer of music on a particular song. Usually for a single instrument.
Think of it in this way: An audio cassette has four tracks, left and right channels for side A and side B. With a four track device, all of these channels are used for side A, and each can be read from or written to separately.