defense mechanism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Biology A physiological reaction of an organism used in self-protection, as against infection.
• noun Psychology Any of various usually unconscious mental processes, including denial, projection, rationalization, and repression, that protect the ego from shame, anxiety, conflict, loss of self-esteem, or other unacceptable feelings or thoughts.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun psychology A psychological strategy used unconsciously by people to cope with harmful or stressful situations.
• noun evolutionary biology An automatic reaction of the body against foreign antibodies.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (psychiatry) an unconscious process that tries to reduce the anxiety associated with instinctive desires