Real Dictionary

overdub in American English
a recording of sounds, music, etc. that is added to or superimposed on another recording
to add (sounds, music, etc.) to (an already existing recording)
intransitive verb: to add other recorded sound or music, as a supplementary instrumental or vocal track, to a taped musical track to complete or enhance a recording
noun: a recorded segment or layer of instrumental parts, vocalization, or sound effects integrated into a musical recording
transitive verb: to add (a track or tracks) to a musical recording

overdub in British English (in multitrack recording)
noun: the addition of new sound to a recording; the blending of various layers of sound in one recording
verb: to add (new sound) on a spare track or tracks