In psychology, libido (lɪˈbiːdoʊ; from the Latin libīdō, "desire") is psychological drive or energy, usually conceived as sexual in nature, but also includes other forms of desire. The term was originally used in psychoanalytic theory, where the neurologist and pioneering psychoanalyst Sigmund Freud began by employing the term in reference to the energy of the sexual drive, later generalising the concept to refer to the fundamental energy of all expressions of love, pleasure, and self-preservation.
In common or colloquial usage, a person's overall sexual drive is often referred to as that person's "libido". In this sense, libido is influenced by biological, psychological, and social factors. Biologically, the sex hormones and associated neurotransmitters that act upon the nucleus accumbens (primarily testosterone, estrogen, and dopamine, respectively) regulate sex drive in humans. Sexual drive can be affected by social factors such as work and family; psychological factors such as personality and stress; also by medical conditions, medications, lifestyle, relationship issues, and age.
Sigmund Freud, who is considered the originator of the modern use of the term, defined libido as "the energy, regarded as a quantitative magnitude... of those instincts which have to do with all that may be comprised under the word 'love'." It is the instinctual energy or force, contained in what Freud called the id, the strictly unconscious structure of the psyche. He also explained that it is analogous to hunger, the will to power, and so on insisting that it is a fundamental instinct that is innate in all humans.
Freud pointed out that these libidinal drives can conflict with the conventions of civilised behavior, represented in the psyche by the superego. It is this need to conform to society and control the libido that leads to tension and anxiety in the individual, prompting the use of ego defenses which channel the psychic energy of the unconscious drives into forms that are acceptable to the ego and superego.