To lower and raise one's head slightly and briefly, especially in greeting, assent, or understanding, or to give someone a signal. She nodded in agreement when asked if she wanted to join the team.
To move one's head up and down repeatedly, often involuntarily, as a sign of drowsiness or sleepiness. He began to nod off during the long lecture.
To give a signal or direction by a nod of the head. The manager nodded towards the door, indicating that it was time to leave.