Polyphony is music consisting of several independent melodic voices, as opposed to music with just one voice (monophony) or music with one dominant melodic voice accompanied by chordss (homophony).
The term is usually used in reference to music of the late Middle Ages and Renaissance - Baroque forms such as the fugue which might be called polyphonic are usually described instead as contrapuntal.
More generally, the word can mean the simultaneous sounding of more than one note; hence, a polyphonic synthesiser is one capable of playing more than one note at a time. Such an instrument capable of playing, say, 16 notes at once is said to have 16 voice polyphony.
