A psychoanalytic term first described by Sigmund Freud. A defense mechanism employed to avoid true emotions. Reaction Formation occurs when person displays the opposite emotions of what he/she is really feeling. The avoidance of pain through psychological defense mechanisms. Can be done subconsciously.
When your girlfriend says, "I can't wait to move back to California." She really means, "I'm going to be completely heartbroken when I leave on December 22nd." By displaying the opposite emotion, she is utilizing reaction formation.