defense mechanism

noun

1. Physiology. the defensive reaction of an organism, as against a pathogenic microorganism.
2. Psychology. an unconscious process, as denial, that protects an individual from unacceptable or painful ideas or impulses.

n.

1. Any of a variety of usually unconscious mental processes used to protect oneself from shame, anxiety, loss of self-esteem, conflict, or other unacceptable feelings or thoughts, and including behaviors such as repression, projection, denial, and rationalization.
2. immunological mechanism

In psychology, a Freudian term referring to an unconscious avoidance of something that produces anxiety or some other unpleasant emotion. For example, someone who blots out the memory of a terrible accident is using a defense mechanism. Regression and sublimation are common defense mechanisms.