“Prana is not something related to breath alone. Breathing is only one of the many exercises through which we get to the real pranayama. The breathing is a manifestation of a vitalizing force called prana. By regulating the physical breathing, the prana is controlled and this process of controlling the subtle prana is called pranayama.
This vital energy is found in all forms of life from mineral to man. Prana is found in all things having life. This prana is not the consciousness or spirit but is merely a form of energy used by the soul in its material and astral manifestations. The whole body is controlled and regulated by the force of prana. Every cell in the body is controlled by it. Prana is in all forms of matter and yet it is not matter. It is the energy or force that animates matter.
Prana is in the air, but it is not the oxygen, nor any of its chemical constituents. It is in food, water, and in the sunlight, yet it is not vitamin, heat or light-rays. Food, water, air, etc., are only the media through which the prana is carried. We absorb this prana through the food we eat, the water we drink, and the air we breathe.
Prana is also known as universal energy. It is the prana that is manifesting itself as gravitation, electricity, as the actions of the body, and as the nerve currents and thought force. From thought down to the lowest physical force, everything is thus the manifestation of prana.
Knowledge and control of prana manifested in individuals is called pranayama, which opens to us the door to almost unlimited power. The control of prana being the one idea of pranayama, all the training and Yogic exercises advocated in Hatha Yoga are for that one end. This little wave of prana, which represents all mental and physical energies, is the easiest to control through the regulation of physical breathing.”
The Complete Illustrated Book of Yoga