mouthing

In sign language, mouthing is the production of visual syllables with the mouth while signing. Although not present in all sign languages, and sometimes not in signers at all levels of education, where it does occur it may be an essential (that is, phonemic) element of a sign, distinguishing signs which would otherwise be homophones; in other cases a sign may seen to be flat and incomplete without mouthing even if it is unambiguous.