prana – pra – usually translated as “forth” (as in “going forth”) and ana – breath or vitality, thus prana can be translated as “the going-forth breath”.
But is prana merely the physical breath? In the Vedas, Prana is personified – Mahaprana – as the first-born and foremost amongst the Devas and sometimes referred to as Prajapati. The Prana Sukta of the Atharva Veda describes Prana as follows:
Reverence unto Prana, to whom all this (universe) is subject, who has become the Lord of All, on whom all is supported. 
Reverence, O Prana, to Thy roaring (wind), reverence, O Prana, to Thy thunder, reverence, O Prana, to Thy lightening, reverence, O Prana, to Thy rain! 
When Prana calls aloud to the plants with His thunder, they are fecundated, they conceive, and then are produced abundant (plants). 
When the season has arrived, and Prana calls aloud to the plants, then everything rejoices, whatsoever is upon the Earth. 
When Prana has watered the great Earth with rain, then the beasts rejoice; (they think): “strength forsooth, we shall now obtain.” 
When they had been watered by Prana, the plants spake in concert: “Thou hast, forsooth, prolonged our life, Thou hast made us all fragrant.” 
Reverence be, O Prana, to Thee coming, reverence to Thee going; reverence to Thee standing, and reverence, too, to Thee sitting! 
Reverence be to Thee, O Prana, when Thou breathest in, reverence when Thou breathest out! Reverence be to Thee when Thou are turned away, reverence to Thee when Thou are turned hither: to Thee, entire, reverence be here! 
Of Thy dear form, O Prana, of Thy very dear form, of the healing power that is Thine, give unto us, that we may live! 
Prana clothes the creatures, as father his dear son. Prana, truly, is the Lord of All, of all that breathes, and does not breathe. 
Prana is death, Prana is fever. The Devas worship Prana. Prana shall place the truth-seeker in the highest world. 
Prana is Viraj, Prana is Deshtri (the guide): all worship Prana. Prana verily is the Sun and the Moon. They call Prana Prajapati. 
The rice and barley are in-breathing and out-breathing. Prana is called a steer. In-breathing forsooth, is founded upon barley; rice is called out-breathing. 
Man breathes out and breathes in when within the womb. When Thou, O Prana, quickenest him, then is he born again. 
They call Prana Matarishvan (the wind); Prana forsooth, is called Vata (the wind). The past and the future, the all, verily is supported upon Prana. 
The holy (atharvana) plants, the magic (angarisa) plants, the divine plants, and those produced by men, spring forth, when Thou, O Prana, quickenest them. 
When Prana has watered the great Earth with rain, then the plants and every sort of herb springs forth. 
Whoever, O Prana, knows this regarding Thee, and on what Thou are supported, to him all shall offer tribute in yonder highest world. 
As all these creatures, O Prana, offer Thee tribute, so they shall offer tribute (in yonder world) to Him who hears Thee, o far-famed One! 
He moves as an embryo within the Devas; having arrived and being in existence, He is born again. Having arisen He enters with His mights, the present and the future,
as a father (goes to) his son. 
When as a swan He rises from the water, he does not withdraw His one foot. If in truth He were to withdraw it, there would be neither today nor tomorrow, no night and no day, never would the dawn appear. 
With either wheels, and one felloe He moves, containing a Thousand sounds (elements), upward in the east, downward in the west. With (His) half He produced the whole world: what is the visible sign of His (other) half? 
He who rules over all this derived from every source, and over everything that moves; reverence to Thee, O Prana, that wieldest a swift bow against others! 
May Prana, who rules over this (all) derived from every source, and over everything that moves, (may He) unwearied, strong through the Brahmâ, adhere to me! 
Erect He watches in those that sleep, nor does lie down across. No one had heard of His sleeping in those that sleep. 
O Prana, be not turned away from me, Thou shalt not be other than myself! As the embryo of the Waters,
Thee, O Prana, do bind to me, that I may live. 
(Trans. Maurice Bloomfield)
Vedic Homologies: Breath and Sacrifice
The Satapatha Brahmana establishes a link between prana and the Sacrificial Fires – equating the Fire with the breath, as the sacrificer establishes the Fire by blowing on it. When the sacrificer inhales, the Fire is established within him. The three Vedic Fires are equated with three breaths: the Ahavaniya (the fire in which offerings are given) is equated with exhalation; the Garhaptya (where offerings are prepared) is the inhalation, and the Daksinagni (where food for Brahmins is prepared) is the circulatory breath.
Also in the Satapatha Brahmana is an explicit connection made between the mind and the pranas – that the mind is the “overlord” of the pranas, and thus that it is through the mind that the pranas can be controlled. There is also mention made of mantras as kindlers of the inner fires.
Prana in the Upanishads
In the later Brahmanas and Upanishads, prana is given further homologies. In the Chandogya Upanishad prana is equated with life-breath:
“Life-breath is one’s father, life breath is one’s mother. life-breath is one’s brother, life-breath is one’s sister, life-breath is one’s teacher, life-breath is the Brahman.”
In the Brhadaranyaka Upanishad prana is identified as being all-pervasive:
“By prana as by a thread, this world, the other world, and all beings are held together.”
Hence prana is identified both as the breath of life in living beings, and the all-pervasive universal breath of life. It is also sometimes identified with Fire, and the all-pervasive Akasa.
Prana and Science: Theosophy, Vivekananda & Mesmerism
Vivekananda’s reworking of Classical Yoga formulations, in works such as Raja Yoga (1896) did much to make Indian ideas compatible with the emerging “scientific” discourse of the late 19th & early 20th century. This was very much part of the larger movement on the part of Indian intellectuals such as the Brahmo Samaj to “modernise” hinduism as a rational, universal, and monotheistic religion.
When Raja Yoga was published, Vivekananda had been living in America for three years, having participated in the World Parliament of Religions in 1893, he stayed in the USA and began to tour and lecture. He was very aware of the need to adapt Indian philosophy to the needs and tastes of the Americans. As he put it himself, “Out of bewildering Yoga-ism must come the most scientific and practical psychology.” In order to re-present Yoga according to the popular science of the period, Vivekananda drew on Samkhya philosophy and Patanjali’s Yoga sutras, and re-interpreted them in terms of the fluid magnetics of Mesmerism and Naturalephilosophie. Vivekananda asserted that “control of prana” was the root of faith-healing, Spiritualism, Hypnotism and Christian Science.
Vivekananda’s view of Prana is thus influenced by the theories of Anton Mesmer, in particular, the notion that a subtle, physical fluid fills the universe and acts as a medium, connecting man, the earth, and the heavenly bodies. Disease, according to this theory, occurrs when the distribution of this fluid around the human body is unequal. Vivekananda goes to some lengths to present mesmeric fluid as “scientific” and to demonstrate that this “science” originated in ancient India. A similar attempt to make connections between magnetism and prana can be found in the writings of the Theosophist, Srinivasa Iyangar, who in his translation of the Hatha Yoga Pradipika (1893) opines that Prana does not merely refer to the air that one breathes, but to a subtle, magnetic current.
Similarly, Vivekananda re-presented the Yogic concept of Samadhi in terms of Self-realization or the realization of “human potential”. He also writes of Kundalini in terms of “psychic prana” – that it was the only way to attain “Divine Wisdom”. Interestingly enough, he is rather dismissive of the physical practices of Hatha Yoga, saying that although they make the body strong, the practices are “very difficult” and “do not lead to much spiritual growth.”
See Elizabeth De Michelis: A History of Modern Yoga (Continuum Int/l) 2005.