1Multiplication of sounds or voices; an instance of this.
In early use especially with reference to an echo..
2Music. The use of polyphony; polyphonic style or composition; an instance of this. rare.
Origin
Early 18th century; earliest use found in William Derham (1657–1735), Church of England clergyman and natural philosopher. From ancient Greek πολύϕωνος having many tones + -ism.