Source : Webster's Revised Unabridged Dictionary (1913)

Counterclaim \Coun"ter*claim`\ (-kl[=a]m`), n. (Law)
   A claim made by a person as an offset to a claim made on him.

Source : WordNet®

counterclaim
     n : a claim filed in opposition to another claim in a legal
         action
     v : set up a claim in opposition to a previous claim