Kundalini is a Sanskrit word meaning either "coiled up" or "coiling like a snake." There are a number of other translations of the term usually emphasizing a more serpent nature to the word - e.g. 'serpent power'.
The caduceus symbol of coiling snakes is thought to be an ancient symbolic representation of Kundalini physiology.
The concept of Kundalini comes from yogic philosophy of ancient India and refers to the mothering intelligence behind yogic awakening and spiritual maturation. It might be regarded by yogis as a sort of deity, hence the occasional capitalization of the term.
Within a western frame of understanding it is often associated with the practice of contemplative or religious practices that might induce an altered state of consciousness, either brought about spontaneously, through a type of yoga, through psychedelic drugs, or through a near-death experience.
According to the yogic tradition Kundalini is curled up in the back part of the root chakra in three and one-half turns around the sacrum. Yogic phenomenology states that kundalini awakening is associated with the appearance of bio-energetic phenomena that are said to be experienced somatically by the yogi.
This appearance is also referred to as "pranic awakening". Prana is interpreted as the vital, life-sustaining force in the body. Uplifted, or intensified life-energy is called pranotthana and is supposed to originate from an apparent reservoir of subtle bio-energy at the base of the spine. This energy is also interpreted as a vibrational phenomena that initiates a period, or a process of vibrational spiritual development.
The source text for the concept of kundalini is the "Hatha Yoga Pradipika" written by Swami Svatmarama (English translation, 1992) somewhere between the twelfth and fifteenth centuries. Any examination of the topic should include this work. The pradipika is one of the later developments in yoga sacred texts. Hatha Yoga is strictly speaking a forcing technique which has as its primary aim the forcing of the arising of kundalini.
The main emphasis is a difficult regime of breathing techniques meant to increase the store of "prana" in the body. The well known physical postures are only meant to be an aid to maintain peak physical fitness, so as to support the real work of the breathing practices. All of this has, according to tradition, to be accompanied by prolonged and unbroken meditation practice (for which the main text is the "Yoga Sutras of Patanjali"). The text adds that great good fortune is another requirement, i.e.luck, for the procedure to succeed. However, these techniques are not without dangers.