A synth whose circuitry is specialized for the purpose of producing string and pad sounds. A number of string synth models were produced in the 1970s; at the time, the circuitry needed to produce a general-purpose polyphonic synthesizer was costly. However, several circuit designers discovered simplified, non-voltage controlled oscillator circuits which could be produced cheaply, and which by happy conincidence were fairly good at imitating the sounds of massed strings — something that performers of the time were looking for in a polyphonic synth. Further, newly developed chorusing circuits helped these circuits produce a more pleasing sound. The typical string synth contained an oscillator circuit for each note (making it fully polyphonic), some very basic filtering to allow for some timbral variation, and a chorus circuit, at a reasonable cost for the day.
String synthesizers fell out of favor in the 1980s when proper polyphonic synthesizers became less expensive. Recently, there has been something of a resurgence in string synths, as performers have discovered certain subtle properties of the vintage units that are difficult to duplicate on other synths. No new string synths have been manufacturered to date, but many vintage units have been restored (they were manufacturered in large numbers, so prices on vintage units aren't too high), and several soft synths have been produced which emulate string synths.