Kundalini literally means coiled. In yoga

Yoga

Yoga is a physical, mental, and spiritual discipline, originating in ancient India. The goal of yoga, or of the person practicing yoga, is the attainment of a state of perfect spiritual insight and tranquility while meditating on Supersoul...

, a "corporeal energy" - an unconscious, instinctive or libidinal

Libido

Libido refers to a person's sex drive or desire for sexual activity. The desire for sex is an aspect of a person's sexuality, but varies enormously from one person to another, and it also varies depending on circumstances at a particular time. A person who has extremely frequent or a suddenly...

 force or Shakti

Shakti

Shakti from Sanskrit shak - "to be able," meaning sacred force or empowerment, is the primordial cosmic energy and represents the dynamic forces that are thought to move through the entire universe in Hinduism. Shakti is the concept, or personification, of divine feminine creative power, sometimes...

, lies coiled at the base of the spine. It is envisioned either as a goddess or else as a sleeping serpent, hence a number of English renderings of the term such as 'serpent power'. The kundalini resides in the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 bone in three and a half coils and has been described as a residual power of pure desire.
Kundalini is described as a sleeping, dormant potential force in the human organism.