Which of these is NOT one of the major personality structures proposed by Sigmund Freud in his psychoanalytic theory of development?

In Freud's psychoanalytic theory of development, the major personality structures are the id, ego, and superego. The id is the primal part of personality that operates on the pleasure principle, seeking immediate gratification. The ego, developing from the id, operates on the reality principle, mediating between the desires of the id and the constraints of reality. The superego represents the internalized moral standards and ideals that we acquire from our parents and society, acting as a counterbalance to the id.

The term "libido," however, refers to the energy derived from the id that drives human behavior and desires, particularly those of a sexual nature. It is not a distinct personality structure like the id, ego, or superego. Understanding these concepts is crucial in grasping Freud's view of human personality development and how various elements interact to shape behavior.