Libido is the name that Jung gives for psychic energy. Just as the physical body has energy, so does the psyche. In Freudian theory, it refers to sexual energy and each person has only a certain amount of libido that can be "cathected" and fixated onto various objects.
List of books: Libido
Other /More definition:
Libido refers to an instinctual pressure for gratification of sexual and aggressive desires.