defense mechanism, in psychoanalysis, any of a variety of unconscious personality reactions which the ego uses to protect the conscious mind from threatening feelings and perceptions. Sigmund Freud first used defense as a psychoanalytic term (1894), but he did not break the notion into categories, viewing it as a singular phenomenon of repression. His daughter, Anna Freud, expanded on his theories in the 1930s, distinguishing some of the major defense mechanisms recognized today. Primary defense mechanisms include repression and denial, which serve to prevent unacceptable ideas or impulses from entering the conscience. Secondary defense mechanisms—generally appearing as an outgrowth of the primary defense mechanisms—include projection, reaction formation, displacement, sublimation, and isolation.

In psychoanalytic theory, an often unconscious mental process (such as repression) that makes possible compromise solutions to personal problems or conflicts. The compromise generally involves concealing from oneself internal drives or feelings that threaten to lower self-esteem or provoke anxiety. The term was first used by Sigmund Freud in 1894. The major defense mechanisms are repression, the process by which unacceptable desires or impulses are excluded from consciousness; reaction formation, a mental or emotional response that represents the opposite of what one really feels; projection, the attribution of one's own ideas, feelings, or attitudes (especially blame, guilt, or sense of responsibility) to others; regression, reversion to an earlier mental or behavioral level; denial, the refusal to accept the existence of a painful fact; rationalization, the substitution of rational and creditable motives for the true (but threatening) ones; and sublimation, the diversion of an instinctual desire or impulse from its primitive form to a more socially or culturally acceptable form. Seealso ego; neurosis; psychoanalysis.

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