Source : Webster's Revised Unabridged Dictionary (1913)

Aphthong \Aph"thong\ (?; 277), n. [Gr. ? silent; 'a priv. + ?
   voice, sound, fr. ? to sound.]
   A letter, or a combination of letters, employed in spelling a
   word, but in the pronunciation having no sound. --
   {Aph*thon"gal}, a.