The act of a member of the Royal Family waving at a member of the public but not in a Royal style (hand rotating in a forward direction with fingers fixed at a horizontal angle) but with their hand and fingers pointing to the person and them moving their fingers up and down in a friendly manner just like they are waving at an acquaintance.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.