The Collaborative International Dictionary of English v.0.48:

hairball \hair"ball`\, hair ball \hair" ball`\(h[^a]r"b[e^]l`),
   n.
   a compact mass of hair that forms in the stomach of animals
   as a result of licking fur; as, the cat coughed up a hairball
   right on the new rug. [WordNet sense 1 + 2]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

hair ball
    n 1: a compact mass of hair that forms in the alimentary canal
         (especially in the stomach of animals as a result of
         licking fur) [syn: hairball, hair ball, trichobezoar]