Prāṇa

Prāṇa (प्राण) is the Sanskrit word for "vital life" (from the root prā). In Vedantic philosophy, prana is the notion of a vital, life-sustaining force of living beings and vital energy, comparable to the Chinese notion of Qi. In human beings, it flows through a network of fine subtle channels called nāḍis.

In the Theosophical literature prāṇa is the third (sometimes second) principle of human beings, usually referred as "life principle" or "vital principle."

In Hinduism

According to Hindu philosophy there are five pranas or vital currents that sustain physiological processes. They are:

Prāṇas

Prāṇa	Responsibility
Prāṇa	Beating of the heart and breathing. Prana enters the body through the breath and is sent to every cell through the circulatory system.
Apāna	elimination of waste products from the body through the lungs and excretory systems
Uḍāna	sound production through the vocal apparatus, as in speaking, singing, laughing, and crying. Also it represents the conscious energy required to produce the vocal sounds corresponding to the intent of the being. Hence Samyama on udana gives the higher centers total control over the body.
Samāna	the digestion of food and cell metabolism (i.e. the repair and manufacture of new cells and growth). Samana also includes the heat regulating processes of the body. Auras are projections of this current. By meditational practices one can see auras of light around every being. Yogis who do special practise on samana can produce a blazing aura at will.
Vyāna	the expansion and contraction processes of the body, e.g. the voluntary muscular system

In Theosophy

Mme. Blavatsky defines prāṇa as follows:

"Prana," or "Life," is, strictly speaking, the radiating force or Energy of Atma -- as the Universal Life and the ONE SELF, -- ITS lower or rather (in its effects) more physical, because manifesting, aspect. Prana or Life permeates the whole being of the objective Universe; and is called a "principle" only because it is an indispensable factor and the deus ex machina of the living man.