As a long time lurker who has never posted, I expect describing my biggest choke is probably as good a place as any for a first-time post, so here it goes...
First, I don't see a single missed shot as a choke. It's only a missed shot. To choke, you've got to feel it as its happening. It happens over time, where you can watch it unfold minute by minute, inning by inning, game by game. Hell, everybody misses a shot every now and then. It takes a real choker to do this: APA Individual Regionals in Nashville this past Fall, I'm an SL 6 and up 4-0 in a race to 5 and... you guessed it, I blow the lead and lose, 4-5.
I could not, to this day, describe any single error. Just one little, unremarkable mistake after another and I could feel the noose getting tighter and tighter and I blew a 4-0 lead.
And that is the definition of "choke."