WordNet
n. (psychiatry) a defense mechanism in which a person unconsciously develops attitudes and behavior that are the opposite of unacceptable repressed desires and impulses and serve to conceal them; "his strict morality is just a reaction formation to hide his sexual drive"
Wikipedia
In psychoanalytic theory, reaction formation is a defensive process ( defense mechanism) in which emotions and impulses which are anxiety-producing or perceived to be unacceptable are mastered by exaggeration ( hypertrophy) of the directly opposing tendency. The reaction formations belong to Level III or neurotic defense mechanisms, which also include intellectualization, dissociation, displacement and repression.
Usage examples of "reaction formation".
Unfortunately, this factor in his personality, which he might have sublimated harmlessly by becoming a social worker, was complicated by unresolved Oedipal hostilities and a reaction formation in favor of 'spying,' which led him to become a policeman.