A small nod of the head, done repetitively in synchronization with music. Denotes approval of music by the listener. Performed at concerts by concert-goers, or while listening to music by the listener.
Substituted for dancing by hipsters, who are too cool to dance.
Andy: Dude, how was the DFA1979 show?
Brad: Great, except for all the douches doing the hipster head bob. I just wanted to get down!