Definify.com

Webster 1913 Edition

(?; 277)

Noun.

[Gr. [GREEK] silent;

ἀ

priv. + [GREEK] voice, sound, fr. [GREEK] to sound.]

A letter, or a combination of letters, employed in spelling a word, but in the pronunciation having no sound.

–

Aph-thon′gal

Adj.

Noun.

[Gr. without, and sound.]

A letter or combination of letters, which, in the customary pronunciation of a word, have no sound.

aphthong ‎(plural aphthongs)

(phonology) A letter or combination of letters employed in spelling a word but not pronounced.
The letters "gh" in "knight" are an aphthong.