In Freudian theory, a defense mechanism is a psychological strategy used to protect oneself from stress, embarrassment or psychological injury. Freud recognized several defense mechanisms which most people use at one time or another during their lives.
Denial is a very well-known defense mechanism. Most people have known someone who is "in denial". An example of denial is the alcoholic who says she can stop drinking any time she wants, as she continues to pour herself glass after glass of wine. Another example is the high school girl who denies that her boyfriend is cheating on her, ...
In this answer, I begin by explaining what Freud meant by the term "defense mechanism". I go on to identify and define three different defense mechanisms, giving examples of how each defense mechanism might be seen in real life.