The Collaborative International Dictionary of English v.0.48:

Pterygium \Pte*ryg"i*um\, n.; pl. E. Pterygiums, L.
   Pterygia. [NL., fr. Gr. ?, properly a dim, akin to ? a
   feather.] (Med.)
   A superficial growth of vascular tissue radiating in a
   fanlike manner from the cornea over the surface of the eye.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pterygium
    n 1: either of two thickened triangular layers of conjunctiva
         extending from the nasal edge of the eye to the cornea; it
         arises from irritation of the pinguecula