Strictly speaking a Charley Horse either refers to a leg cramp or hitting someone above or to the side of their knee to cause said leg cramp... though some also use it for the "kneeing someone in the back of their knee" maneuver even when no cramping is induced.
Other names include dead-legging (which can also refer to an induced cramp), jelly-legging, lawnchairing, tabling, folding, etc. I just picked Charley Horse because it has "horse" in it.
