Source : Webster's Revised Unabridged Dictionary (1913)

Mydriasis \My*dri"a*sis\, n. [L., fr. Gr. ?.] (Physiol. & Med.)
   A long-continued or excessive dilatation of the pupil of the
   eye.

Source : WordNet®

mydriasis
     n : reflex pupillary dilation as a muscle pulls the iris
         outward; occurs in response to a decrease in light or
         certain drugs