Source : Webster's Revised Unabridged Dictionary (1913)

Heteronym \Het"er*o*nym\, n.
   That which is heteronymous; a thing having a different name
   or designation from some other thing; -- opposed to homonym.

Source : WordNet®

heteronym
     n : two words are heteronyms if they are spelled the same way
         but differ in pronunciation (e.g. `bow')