Source : Webster's Revised Unabridged Dictionary (1913)

Astigmatism \A*stig"ma*tism\, n. [Gr. 'a priv. + ?, ?, a prick
   of a pointed instrument, a spot, fr. ? to prick: cf. F.
   astigmatisme.] (Med. & Opt.)
   A defect of the eye or of a lens, in consequence of which the
   rays derived from one point are not brought to a single focal
   point, thus causing imperfect images or indistinctness of
   vision.

   Note: The term is applied especially to the defect causing
         images of lines having a certain direction to be
         indistinct, or imperfectly seen, while those of lines
         transverse to the former are distinct, or clearly seen.

Source : WordNet®

astigmatism
     n 1: (ophthalmology) impaired eyesight resulting usually from
          irregular conformation of the cornea [syn: {astigmia}]
     2: (optics) defect in an optical system in which light rays
        from a single point fail to converge in a single focal
        point [syn: {astigmia}] [ant: {stigmatism}]