astigmatism

n. [ Gr. 'a priv. + &unr_;, &unr_;, a prick of a pointed instrument, a spot, fr. &unr_; 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. [ 1913 Webster ]

☞ 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. [ 1913 Webster ]