noun
Refers specifically to the hair above the upper lip, as opposed to a full beard (あごひげ) or sideburns (ほおひげ).
彼は口ひげを生やしている。
He has a moustache.
口ひげを剃るのを忘れた。
I forgot to shave my moustache.
Rare kanji form; 髯 specifically refers to a beard or whiskers.
Compound of 口 (kuchi, 'mouth') and ひげ (hige, 'facial hair'). The reading くちひげ is a straightforward combination.