\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.