Mustache is the usual American spelling of moustache: the hair a person grows on the upper lip, between the nose and the mouth. It is not a beard, which is the hair on the chin and cheeks — though actually, in loose talk many people say beard for all face hair, and listeners often still follow. British English mostly writes moustache; the thing itself is the same. A big, curly, old-fashioned one is sometimes called a mustachio, which sounds a bit comic or grand. The word is neutral: fine in a joke, a novel, or a doctor's note.