Final answer:
Freud defined defense mechanisms as unconscious thought processes that alleviate anxiety by concealing one's true feelings, often through behaviors that appear opposite to one's actual desires or sentiments.
Step-by-step explanation:
A defense mechanism is best defined by Freud as a thought process that operates at an unconscious level to help an individual reduce anxiety.
These unconscious protective behaviors are a means for the ego to cope with internal conflicts, usually between the id's desires and the superego's moral constraints, without the individual's conscious awareness.
Common types of defense mechanisms include repression, where anxiety-inducing memories are blocked from consciousness, reaction formation, where an individual exhibits the opposite behavior to their true feelings, and displacement, where feelings are redirected towards a less threatening target.