1. Biology. A physiological reaction of an organism used in self-protection, as against infection.
2. 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.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.