The Medieval period includes music from after the fall of Rome to about 1400. Monophonic chant, also called plainsong or Gregorian chant, was the dominant form until about 1100. Polyphonic (<answer>multi-voiced</answer>) music developed from monophonic chant throughout the late Middle Ages and into the Renaissance, including the more complex voicings of motets.
What does Polyphonic mean? 
multi-voiced