The Collaborative International Dictionary of English v.0.48:

Myosis \My*o"sis\, n. [NL., fr. Gr. ? to close the eyes or
   lips.] (Med.)
   Long-continued contraction of the pupil of the eye.
   [1913 Webster]

WordNet (r) 3.0 (2006):

myosis
    n 1: an acute infectious disease occurring in epidemic form and
         featuring paroxysms of pain (usually in the chest) [syn:
         epidemic pleurodynia, epidemic myalgia, myosis,
         diaphragmatic pleurisy, Bornholm disease]
    2: reflex contraction of the sphincter muscle of the iris in
       response to a bright light (or certain drugs) causing the
       pupil to become smaller [syn: light reflex, pupillary
       reflex, miosis, myosis]