An anime/manga slang term for a character that is cold, blunt, cynical, and pretty much doesn't care if her beloved dies. That's what she is on the outside but she is actually caring and nice on the inside.
Differs from tsundere since tsundere is when the character frequently runs hot and cold between tsun and dere. Kuudere is when the character only occasionally shows her caring side.
C.C. - The kuudere of Code Geass