Part (music)
A group of singers who are all singing the tune together are singing in unison.  If they divide into soprano, alto, tenor and bass, they are singing in four parts.  The sopranos may be singing the tune and the altos, tenors and basses are providing the harmony underneath.  This is called “4 part harmony”.  If each of the four parts has its own share of the melody (or its own melody) it is polyphonic music.  Polyphonic music is always in several parts.