pto·sis 
(tōsĭs)
(tōsĭs)n. pl. pto·ses (-sēz)
Abnormal lowering or drooping of an organ or a part, especially a drooping of the upper eyelid caused by muscle weakness or paralysis.
[Greek ptōsis, fall, from piptein, ptō-, to fall; see pet- in the Appendix of Indo-European roots.]
ptotic (-tĭk) adj.