A*stig"ma*tism (#), n. [Gr. 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 indistictness of vision.
⇒ 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.
© Webster 1913.