Source : Webster's Revised Unabridged Dictionary (1913)

Polyphone \Pol"y*phone\, n.
   A character or vocal sign representing more than one sound,
   as read, which is pronounced r[=e]d or r[e^]d.

Source : WordNet®

polyphone
     n : a letter that has two or more pronunciations; "`c' is a
         polyphone because it is pronounced like `k' in `car' but
         like `s' in `cell'" [syn: {polyphonic letter}]