The Collaborative International Dictionary of English v.0.48:

Aphasia \A*pha"si*a\, Aphasy \Aph"a*sy\, n. [NL. aphasia, Gr. ?,
   fr. ? not spoken; 'a priv. + ? to speak: cf. F. aphasie.]
   (Med.)
   Loss of the power of speech, or of the appropriate use of
   words, the vocal organs remaining intact, and the
   intelligence being preserved. It is dependent on injury or
   disease of the brain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aphasia
    n 1: inability to use or understand language (spoken or written)
         because of a brain lesion