A creature that combines characteristics of cat (usually ears and a tail) and a large-breasted young woman. Their very existence draws the wrath of god-- who has sworn to himself to smite a catgirl every time someone bring rationality into an irrational setting ('cause god doesn't like people ruining his suspension of belief either.) Most commonly this is by trying to bring physics into a fantasy world.
Remember kids, every time you try to drag real life physics into a made-up setting, God kills a catgirl. So do it at every possible opportunity!!!