"Pranayama" comes from two Sanskrit words: "prana", meaning the fundamental life force, and "yama" meaning to control. Pranayama is, therefore channeling or controlling the life force. "Pranayama" can also be seen as the combination of "pran", the life force, with "ayama", meaning expansion. In this sense pranayama expands the life force though all levels of our being, physical, psychological and spiritual.
This is the definition of Pranayama in the Yoga-sutras of PATANJALI:
Tasmin sati svasaprasvasayor-gativicchedah pranayamah - “Regulation of breath or the control of Prana is the stoppage of inhalation and exhalation, which follows after securing that steadiness of posture or seat.”
‘Svasa’ means inspiratory breath. ‘Prasvasa’ means expiratory breath. You can take up the practice of Pranayama after you have gained steadiness in your Asana (seat). If you can sit for 3 hours in one Asana, continuously at one stretch, you have gained mastery over the Asana. If you are able to sit from half to one hour even, you can take up the practice of Pranayama.You can hardly make any spiritual progress without the practice of Pranayama.
The energy in the body is Prana. By controlling the motion of the lungs or respiratory organs, we can control the Prana that is vibrating inside. By control of Prana, the mind can be easily controlled, because the mind is fastened to the Prana, like the bird to the string.
The first important step is to master the Asana of posture or to control the body. The next exercise is Pranayama. Correct posture is indispensably requisite for the successful practice of Pranayama. An easy comfortable posture is Asana. That pose is the best which continues to be comfortable for the greatest length of time.
Chest, neck, and head must be in one vertical line. You should not bend the body either forwards or laterally, either on the right or left side. You should not sit crooked. You should not allow the body to collapse. You must not bend the body either forwards or backwards. By regular practice the mastery over the pose will come by itself.
Fatty people will find it difficult to practice the Padma Asana or the Lotus Pose. They can sit on the Sukha Asana (comfortable pose) or Siddha Asana (perfected pose). You need not wait for practicing Pranayama till you get full mastery over the Asana. Practice Asana and side by side you can practice Pranayama also. In course of time, you will acquire perfection in both. Pranayama can also be practiced by sitting in the chair erect.
In Bhagavad-Gita, the Immortal Song of Lord Krishna, you will find a beautiful description of seat and pose: “In a pure secret place by himself established in a fixed seat of his own, neither too high nor too low, with cloth, black antelope-skin and Kusa grass one over the other, there, making the mind one-pointed, with thought and the functions of the senses controlled, steady on his seat, he should practice Yoga for the purification of the Self, holding the body, head and neck erect, firm, gazing steadily at the tip of the nose without looking around” (Ch. VI—10,11, 12).
Pranayama is the control of the Prana and the vital forces of the body. It is regulation of the breath. This is the most important step. The aim of Pranayama is the control of Prana. Pranayama begins with the regulation of the breath for having control over the life-currents or inner vital force. In other words, Pranayama is the perfect control of the life-currents through control of breath. Breath is external manifestation of the gross Prana. A correct habit of breathing must be established by the regular practice of Pranayama. In ordinary worldly persons the breathing is irregular.
If you can control the Prana you can completely control all the forces of the Universe, mental and physical. The Yogi can also control the Omnipresent manifesting power out of which all energies take their origin, whether concerning magnetism, electricity, gravitation, cohesion, nerve-currents, vital forces or thought-vibrations, in fact the total forces of the Universe, physical and mental.
Regular practice of Pranayama bring good appetite, cheerfulness, handsome figure, good strength, courage, enthusiasm, a high standard of health, vigour and vitality and good concentration of mind. Pranayama is quite suitable for the Westerners also. A Yogi measures the span of his life not by the number of years but by the number of his breaths. You can take in a certain amount of energy or Prana from the atmospheric air along with each breath.
Vital capacity is the capacity shown by the largest quantity of air a man can inhale after the deepest possible exhalation. A man takes fifteen breaths in a minute. The total number of breaths comes to 21,600 times per day.