To move your head down and then up, sometimes several times, especially to show agreement, approval, or greeting, or to show something by doing this. (Cambridge Dictionary)
A movement up and down with the head. (Cambridge Dictionary)
Lower and raise one’s head slightly and briefly, especially in greeting, assent, or understanding, or to give someone a signal. (Oxford Dictionaries)
To make a quick downward motion of the head whether deliberately (as in expressing assent or salutation) or involuntarily (as from drowsiness) (Merriam – Webster)
If you nod, you move your head downwards and upwards to show that you are answering ‘yes’ to a question, or to show agreement, understanding, or approval. (Collins Dictionary)
Is that a yes or a no when you nod your head?
What’s the meaning of the phrase ‘A nod is as good as a wink’?
To a person who is ready to understand or undertake something, any subtle signalling of it is sufficient.