Full Definition of INSTRUMENT. 1. : a device used to produce music; also: a singing voice. 2. a: a means whereby something is achieved, performed, or furthered b: one used by another as a means or aid: dupe, tool.ynonym Discussion of INSTRUMENT. instrument, tool, and utensil mean a device for doing work. instrument is used for a device that can be used to do complicated work. tool is used for a device used for a particular job and often suggests that a special skill is needed to use it.