Real Dictionary

nod
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.

nod in American English
a nodding, as of the head, treetops, etc.
to bend (the head) forward slightly and raise it again quickly
to bend the head forward slightly and raise it again quickly, as a sign of greeting, command, acknowledgment, invitation, or, specif., of agreement or assent
intransitive verb: to make a slight, quick downward bending forward of the head, as in assent, greeting, or command
noun: the land east of Eden where Cain went to dwell. Gen. 4:16
transitive verb: to bend (the head) in a short, quick downward movement, as of assent or greeting

nod in British English
verb: to lower and raise (the head) briefly, as to indicate agreement, invitation, etc