Tongue-in-cheek unit of measurement for Gainaxing in anime: One Misty May is equal to a full bounce.
Named for a bouncy girl in a Playboy Bunny suit from the Gainax production Otaku no Video (pictured at right).
See also ADV's "jiggle meter" feature in their DVD release of Plastic Little.
Oh, and in Japan, it's not known as Kasumi Haruka; and it doesn't refer to that beach volleyball player either, nor a certain villain from a certain H-Game. Most definitely not to be confused with Minnie May (though they were both created by the same guy).