In psychology, libido is psychic drive or energy, usually conceived as sexual in nature, but sometimes conceived as including other forms of desire. The term libido was originally used by the neurologist and pioneering psychoanalyst Sigmund Freud who began by employing it simply to denote sexual desire. Over time it came to signify the psychic energy of the sexual drive, and became a vital concept in psychoanalytic theory. Freud's later conception was broadened to include the fundamental energy of all expressions of love, pleasure, and self-preservation.