Source : Webster's Revised Unabridged Dictionary (1913)

Ptosis \Pto"sis\, n. [NL., fr. Gr. ? a falling.] (Med.)
   Drooping of the upper eyelid, produced by paralysis of its
   levator muscle.

Source : WordNet®

ptosis
     n : drooping of the upper eyelid caused by muscle paralysis and
         weakness
     [also: {ptoses} (pl)]