The Collaborative International Dictionary of English v.0.48:

hyperactivity \hyperactivity\ n.
   an unusually high level of activity; -- used especially with
   respect to children who move around frequently and do not sit
   still very long, most noticeably in school. It is sometimes
   associated with attention deficit disorder.
   [PJC]

WordNet (r) 3.0 (2006):

hyperactivity
    n 1: a condition characterized by excessive restlessness and
         movement