Polyphony, from the Greek: “many-sounding”, commonly refers to music where two or more strands are played or sung simultaneously (as opposed to “monophonic”: “single-sounding"). The parts of the music move independently of each other, but still remain as an harmonic whole.
The polyphonic era in Western music is considered to be during the late medieval and Renaissance periods. The late Renaissance, with the choral works of Palestrina, Lassus and others is thought of as the polyphonic “golden age”.
With electronic synthesisers polyphony refers to the ability to play more than a single note simultaneously.