ptosis

from WordNet (r) 3.0 (2006)

ptosis
    n 1: drooping of the upper eyelid caused by muscle paralysis and
         weakness

from The Collaborative International Dictionary of English v.0.48

Ptosis \Pto"sis\ (t[=o]"s[i^]s), n. [NL., fr. Gr. ptw^sis a
   falling.] (Med.)
   Drooping of the upper eyelid, produced by paralysis of its
   levator muscle.
   [1913 Webster]