a sort of hello shake of the head

In our jurisdiction, when a gowned barrister leaves the courtroom while a case is in progress before a judge or judges, he stops before the double doors before leaving, turns and nods with his entire body as a sign of deference and regret for any distraction caused by his exit.

When the proceedings appear not quite so solemn, the nod is merely a dropping of the head, or, more precisely, a south nod.

I sometimes nod to people in the street when they make eye contact with me, even if I don't know them. I suppose I think they might know me and I don't want to be remote. This nod is also a south nod, a nod of respect, or recognition, or salutation, or perhaps all three at once.