The Collaborative International Dictionary
Aphthong
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.
Wiktionary
aphthong
n. (context phonology English) A letter or combination of letters employed in spelling a word but not pronounced.