Prana is the vital air or life’s breath. According to older yogic theory there are five breaths or pranas. The breathing breath is called prana; the breath that goes downward out the anus is called apana (but sometimes apana is used to refer to the “out-breath” in contrast to the “in-breath” of prana); the digestive breath is called samana; the breath that is diffused throughout the whole body is called vyana; and the breath that goes up the throat and enters into the head is called udana. These five breaths, or pranas, resemble the humors of earlier Western medicine.
Further reading: Benimadhab Barua, A History of Pre-Buddhistic Indian Philosophy (Delhi: Motilal Banarsi-dass, 1970); Swami Naranjananda Saraswati, Prana, Pranayama, Prana Vidya (Munger: Bihar School of Yoga, 1994).
Encyclopedia of Hinduism. A. Jones and James D. Ryan. 2007.