Libido in its common usage means sexual desire, however more technical definitions, such as found in the work of Carl Jung, are more general, referring to libido as the free creative, or psychic, energy an individual has to put toward personal development, or individualization.

Psychology

Sigmund Freud introduced the term and pointed out that libido is the instinctual energy or force that can come into conflict with the conventions of civilized behavior. It is the need to conform to society and control the libido, contained in what Freud defined as the Id, that leads to tension and disturbance in both society and the individual. This disturbance Freud identified as neurosis.

Libido is generally considered synonymous with such concepts as élan vital and psychophysiological energy; related concepts from Eastern philosophy include Kundalini and Tantra.

Libido can also be classified as the urge to create life. Naturally for humanity the natural way through which this occurs is through sex. However at a deep subconscious level, the two can be merged as one, given the reason of sexual attraction and sexual desire.

Physiology

Physicians and psychiatrists consider reductions in libido to be a type of sexual dysfunction and treat it as a medical problem. For example, decreases in libido are linked to decreases in naturally produced estrogen (in women) or testosterone (in both men and women). Hormone deficiencies that cause libido decrease are treated by hormone replacement therapy.

Many medical conditions or treatments also cause of libido decrease. Surgery, fatigue, psychiatric conditions (such as depression or anxiety), and pain can lead to lower libido. Some medications also produce drops in libido (such as SSRIs).

Libido decrease is also associated with aging.

from Wikipedia