The Collaborative International Dictionary of English v.0.48:

Quadrivium \Quad*riv"i*um\, n. [L.]
   The four "liberal arts," arithmetic, music, geometry, and
   astronomy; -- so called by the schoolmen. See Trivium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quadrivium
    n 1: (Middle Ages) a higher division of the curriculum in a
         medieval university involving arithmetic and music and
         geometry and astronomy