An acquaintance of sorts, someone you see every so often, but don't actually talk to. Often someone you bump into at work, or a neighbour, or a friend's friend. You wouldn't automatically greet or address her (or him), but the cultural norm dictates some kind of acknowledgement. So, you nod at some complexly-determined degree of vehemence, possibly also twitching your eyebrows. He or she is expected to nod back, offering due recognition.
On Everything, you're more likely to have a noding acquaintance.