Nod of aknowledgement

a nod of the head (either a swift upward jerk of the head, or a slower, downward nod of the head) sometimes exchanged between African-Americans (particularly males, regardless of age), given in acknowledgement of each other's presence and the presumed, shared struggles of being an African-American man. Prevalance of the nod varies greatly from city to city or geographic region.

Non African-American companion upon observing the Nod: "Do you know that guy? Why did you two just do that?"
African-American man: "No, I've never seen him before. I was just giving him a 'Nod of aknowledgement'. We don't all know each other; it's just a form of respect."