European secular unaccompanied vocal music form of the 1500s and 1600s (8)
I believe the answer is:
madrigal
(Other definitions for madrigal that I've seen before include "Part song for unaccompanied voices" , "Unaccompanied song in several parts" , "Drag Liam around to hear an unaccompanied song" , "Old part song" , "Unaccompanied harmonised part song" .)