A defensive mechanism was an unconscious strategy that people would use to avoid unwanted thoughts, emotions, or feelings, or to reduce anxiety, distress, or internal conflict. They took many forms, such as aggression and making a joke out of everything. (VOY: "Displaced")
In 2154, Doctor Phlox theorized that Orion pheromones acted as a defense mechanism against competition for males, after several women on Enterprise NX-01 reported headaches and listlessness following exposure to the substance. (ENT: "Bound")
In 2369, the neurosomatic technique was used by Suna when he attempted to extract information from Starfleet officer William T. Riker's memory. During the procedure, Riker's mind created a defense mechanism which resisted the technique. (TNG: "Frame of Mind")
In 2373, Lyris explained Harry Kim's instinctive hostility towards the Nasari as being a defense mechanism programmed into all Taresian children. (VOY: "Favorite Son")