Non-manual signals or NMS for short (also, non-manual markers or NMMs) are grammatical and semantic features other than hands. They include mouth morphemes, eye gazes, facial expressions, body shifting, and head tilting.
ASL sign for NMS.
Non-manual signals are used to modify verbs, adjectives, and other adverbs. They also convey grammatical information such as topicalization, sentence types (wh-question, rh-question, yes/no question, etc.), etc.
In phonology, NMS is the fifth part of signs. A sign or signed word consists of the five smallest parts called parameters: handshape, movement, location, palm orientation, and non-manual signal.
Some signs require NMs in order to be produced correctly. For example, the signs LATE and NOT-YET both have the same handshape, movement, location, and palm orientation. But, they have a different non-manual marker.
Related posts: mouth morphemes.
Phonology: NMS parameter.