to let the head fall slightly forward with a sudden, involuntary movement when sleepy. to doze, especially in a sitting position:

to move the head down and then up again quickly, esp. to show agreement, approval, or greeting: [ I/T ] The teacher nodded (his head) in agreement.

1. transitive verb/intransitive verb [no passive]. If you nod, you move your head downward and upward to show that you are answering `yes' to a question, or to ...