Prana (प्राण, IAST: prāṇa) is a Sanskrit word that refers to a vital, life-sustaining force of living beings and vital energy in natural processes of the universe. Prana is a central concept in Ayurveda and Yoga where it is believed to flow through a network of fine subtle channels called nadis. The three main channels are: the ida, the pingala, and the sushumna. Ida relates to the left side of the body terminating at the left nostril and pingala to the right side of the body terminating at the right nostril. In some practices, alternate nostril breathing balances the prana that flows within the body. When prana enters a period of uplifted, intensified activity, the Yogic tradition refers to it as Pranotthana. 
Prana was first expounded in the Upanishads, where it is part of the worldly, physical realm, sustaining the body and the mother of thought and thus also of the mind. Prana suffuses all living form but is not itself the Atma or individual soul.
The popular understanding of prana as being the same as air is a misunderstanding, or a simplification of the concept. The incorrect assumption that prana is respiratory air arises from the popular understanding of the practice of pranayama, in which the control of prana is achieved (initially) from the control of one's breathing. According to Yogic philosophy the breath, or air, is merely a gateway to the world of prana and its manifestation in the body. In yoga, pranayama techniques are used to control the movement of these vital energies within the body, which is said to lead to an increase in vitality in the practitioner. However, the practice of these techniques is not trivial, and Kason (2000)  mentions circumstances where pranayama techniques might disrupt the balance of a person's life. The possibility of adverse effects resulting from these techniques must therefore not be underestimated.
In practical terms, prana can be explained in various ways. Feelings of hunger, thirst, hot, cold, etc. in the body could, according to this worldview, be interpreted as pranic manifestations. All physical feelings or energies that arise or flow within the body might also be interpreted as evidence that prana is at work. The presence of prana is said to be what distinguishes a living body from a dead one. When a person (or any other living being such as an animal) dies, the prana, or life force, is thought to leave the body through one of several orifices.
Prana is also a term which can be further classified into subcategories, referred to as pranas. According to Hindu philosophy these are the vital principles of basic energy and subtle faculties of an individual that sustain physiological processes. There are five pranas or vital currents in the Hindu system : Prana, Apana, Vyana, Udana and Samana. The pranas constitute the second sheath (kosa) of a human being (who is essentially the Atman or the Self). The koshas are listed below.
- Annamayakosha (Gross Body)
- Pranamayakosha (Vital Air Sheath)
- Manomayakosha (Mental Sheath)
- Vigyanamayakosha (Intellectual Sheath)
- Anandmayakosha or Karanamayakosha (Causal Sheath)
Veerya or seminal energy is said to be "the Prana of Pranas".
- Sovatsky, Stuart (1998) Words from the Soul: Time, East/West Spirituality, and Psychotherapeutic Narrative. Suny Series in Transpersonal and Humanistic Psychology, New York: State University of New York Press.
- Kason, Yvonne (2000) Farther Shores: Exploring How Near-Death, Kundalini and Mystical Experiences Can Transform Ordinary Lives. Toronto: HarperCollins Publishers; Revised edition.