concept originated by Sigmund Freud (Freud, Sigmund) to signify the instinctual physiological or psychic energy associated with sexual urges and, in his later writings, with all constructive human activity. In the latter sense of eros, or life instinct, libido was opposed by thanatos, the death instinct and source of destructive urges; the interaction of the two produced all the variations of human activity. Freud considered psychiatric symptoms the result of misdirection or inadequate discharge of libido.

Carl Jung (Jung, Carl) used the term in a more expansive sense, encompassing all life processes in all species. Later theories of motivation have substituted for libido such related terms as drive and tension.