(Sanskrit; Tibetan bindu) Life-principle; the breath of life; energy. The vital breath, which sustains life in a physical body; the primal energy or force, of which other physical forces are manifestations. In the books of Yoga, prana is described as having five modifications, according to its five different functions. These are: prana (the vital energy that controls the breath), apana (the vital energy that carries downward unassimilated food and drink), samana (the vital energy that carries nutrition all over the body), vyama (the vital energy that pervades the entire body), and udana (the vital energy by which the contents of the stomach are ejected through the mouth). The word Prana is also used as a name of the Cosmic Soul, endowed with activity.

"Prana is the Great Breath. It is the Cosmic Christ. Prana is the life that palpitates within every atom, as it palpitates in every sun. Fire burns because of Prana. Water flows because of Prana. Wind blows because of Prana. The sun exists because of Prana; the life we have is Prana. Nothing can exist in the Universe without Prana. It is impossible for the most insignificant insect to be born, or for the smallest flower to bloom without Prana. Prana exists in the food that we eat, in the air that we breathe, and in the water that we drink. Prana exists within everything. When seminal energy is refined and totally transformed, the nervous system is provided with the richest type of Prana. This rich Prana is deposited within the brain in the form of pure Christic energy, the Wine of Light. An intimate connection exists between the mind, the Prana and the semen. We can gain dominion over the mind and Prana by controlling the seminal energy with the force of willpower. Those people who spill the semen can never gain control over their mind, let alone Prana. Their efforts to gain control over their mind and over Prana will undoubtedly fail. People who gain sexual control, also gain control of their minds and control of their Prana. These types of human beings reach true liberation." - from The Yellow Book

From Kundalini Yoga: The Mysteries of the Fire : “Prana is life and it circulates throughout all of our organs.”  “Prana circulates throughout all of our nadis and vital canals.”  “The sexual energy is Prana, life.”  “Prana is sexual.  The sexual energy is solar.  The solar energy is Christic.  The Cosmic Christ is the Solar Logos.  The solar energy comes from the Cosmic Christ.  The Christic Prana makes the spike of wheat to grow; thus, the Christonic substance ready to be devoured remains enclosed within the grain. The water from the mountain glaciers penetrates within the stump to ripen the grape, within which the whole life, the whole Prana from the Sun-Christ remains enclosed.”