The Collaborative International Dictionary of English v.0.48:

Agraphia \A*graph"i*a\, n. [Gr. 'a priv. + ? to write.]
   The absence or loss of the power of expressing ideas by
   written signs. It is one form of aphasia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

agraphia
    n 1: a loss of the ability to write or to express thoughts in
         writing because of a brain lesion [syn: agraphia,
         anorthography, logagraphia]