Failing to close out a match or point you should have won, typically under pressure. Missing easy shots when serving for the match, double-faulting on match point, or losing a set after being up 5-1. Every player has choked at some point. The best ones learn from it. The term is brutal but accurate: the pressure literally tightens your muscles and shortens your breathing.
"She was serving for the match at 5-3 and lost four straight games. That's a choke. It happens to everyone, but it still stings."