Acronym for "Synthesizer Expansion Module", a product developed by Oberheim in 1973. The SEM contained two VCOs, a multimode VCF, a VCA, and a LFO. Most parameters were controllable by external control voltage inputs. The SEM had no keyboard of its own, being intended to be played by control voltage and gate signals from another analog synthesizer.
The SEM was originally intended to be used to add capability to other synths. However, Oberheim soon realized that they could use a bank of SEMs, with a scanning keyboard, to construct a polyphonic synth of sorts. The result was the Four Voice series. Ironically, far more SEMs were sold integrated into Two/Four/Eight voice synths than were sold as expanders.