A circuit board containing all of the circuitry required for one or more voices of a polyphonic synth. In the 1980s particularly, a number of polyphonic synths were designed with voice cards that plugged into a chassis that contained the circuitry that was common to all voices, such as the voice mixer and output amplifier. Performers could buy a minimally-configured synth (to save up-front money) and then add voice cards later, to increase the number of voices available. It also made servicing easier since a synth with a faulty voice could be repaired simply by swapping the voice card out, a feature appreciated by touring musicians.
Peter Baumann's order of a set of voice cards from E-mu in 1978, for a synth he planned to build, famously led in a convoluted manner to the development of the Emulator.