A monophonic synthesizer or monosynth is a synthesizer that produces only one note at a time, making it smaller and cheaper than a polysynth, a synthesizer which can play multiple notes at once; most later synthesizers are polysynths.
Well-known monosynths include the Minimoog, Roland SH-09 and the Korg Prophecy.
Electropop group Joy Electric released a song entitled "Monosynth" on their album Robot Rock.