What are the defense mechanisms?
There are multiple definitions of defense mechanism based on different theories. Generally, defense mechanisms are human responses responses to anxiety and how the consciousness and unconscious manage the stress of a social situation. Some of the most common defense mechanisms include:

Repression: forcing a feeling from the consciousness to the unconscious because it is seen as socially unacceptable

Regression: falling back into an early state of mental/physical development seen as less demanding and safer

Projection: possessing a feeling that is deemed as socially unacceptable and instead of facing it, that feeling or "unconscious urge" is seen in the actions of other people

Reaction formation: acting the opposite way that the unconscious instructs a person to behave, "often exaggerated and obsessive". For example, if a wife is infatuated with a man who is not her husband, reaction formation may cause her to – rather than cheat – become obsessed with showing her husband signs of love and affection.

Sublimation: seen as the most acceptable of the mechanisms, an expression of anxiety in socially acceptable ways