The Collaborative International Dictionary of English v.0.48:

Hemiopia \Hem`i*o"pi*a\, Hemiopsia \Hem`i*op"si*a\, n. [NL., fr.
   Gr. ? half + Gr. ? sight.] (Med.)
   A defect of vision in consequence of which a person sees but
   half of an object looked at.
   [1913 Webster]