homograph

from WordNet (r) 3.0 (2006)

homograph
    n 1: two words are homographs if they are spelled the same way
         but differ in meaning (e.g. fair)

from The Collaborative International Dictionary of English v.0.48

Homograph \Hom"o*graph\, n. [Gr. "omo`grafos with the same
   letters; "omo`s the same + gra`fein to write.] (Philol.)
   One of two or more words identical in orthography, but having
   different derivations and meanings; as, fair, n., a market,
   and fair, a., beautiful.
   [1913 Webster]