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."
This is the definition of Pranayama in the Yoga Sutras of Pantanjali.
'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.
Prana is Vyasthi, when the individual is concerned. The sum total of the cosmic energy or cosmic Prana is Hiranyagarbha who is known as the floating 'Golden-Egg'. Hiranyagarbha is Samashti Prana. One match stick is Vyashti (single). The whole match box is Samashti. A single mango-tree is Vyashti. The whole mango grove is Samashti. The energy in the body is Prana. By controlling the motion of 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. Just as the bird that is tied to a post by a string, after flying here and there, finds its resting place in the post, so also this mind-bird after running hither and thither, in various sensual objects, finds its resting place during deep sleep in the Prana.
PRANAYAMA (ACCORDING TO THE GITA)
Apane juhvati pranam prane'panam tathapare; Pranapanagatee ruddhva pranayamaparayanah (Gita, Ch. IV-29). Others offer Prana (outgoing breath) in Apana (incoming breath) and Apana in Prana, restraining the passage of Prana and Apana, absorbed in Pranayama. Pranayama is a precious Yajna (sacrifice). Some practise the kind of Pranayama called Puraka (filling in). Some practise the kind of Pranayama called Rechaka (emptying). Some are engaged in the practice of Pranayama called Kumbhaka, by impeding the outward passage of air, through the nostrils and the mouth, and by impeding the inward passage of the air, in the opposite direction.
PRANAYAMA (ACCORDING TO SRI SANKARACHARYA)
"Pranayama is the control of all the life-forces by realising naught but Brahman in all things as the mind, etc.
"The negation of the Universe is the outgoing breath. The thought 'I am Brahman' itself is called the incoming breath.
"The permanence of that thought thereafter is the restrained breath. This is the Pranayama of the wise, while the pressing of the nose is only for the unknown." (Aparokshanubhuti, 118-120)
PRANAYAMA (ACCORDING TO YOGI BHUSUNDA)
Bhusunda says to Sri Vasishtha: "In the cool lotus of the heart within this visible tenement of flesh composed of the five elements, there are two Vayus, viz., Prana and Apana, commingled in it. For those who tread smoothly and without any of the slightest efforts, the path of these two Vayus will become the sun and the moon themselves in the heart-Akasa, and will rove in the Akasa and yet be animating and carrying their fleshy-tabernacle. These Vayus will go up and down to higher and lower states. They are of the same nature in the waking, dreaming and dreamless sleeping state, and permeate all throughout. I am moving in the direction of those two Vayus and have rendered nil all my Vasanas in the waking state lit unto those of the dreamless sleeping state. Divide a filament of the lotus-stalk into a thousand times and you will find these Vayus more subtle than that. Hence it is difficult for me to treat about the nature of these Vayus and their vibrations. Of these, Prana does ceaselessly vibrate in this body with an upward motion, both externally and internally, while Apana, having the same fluctuating tendency, vibrates both external and internal to the body having a downward motion. It will be beneficial if the Prana, exhaled to the extent of 16 digits, is inhaled to the same extent. Only 12 digits are inhaled ordinarily. Those who have brought to experience-viz., the equalisation of Prana in exhalation and inhalation will enjoy infinite bliss.
"Now hear about the characteristics of Prana. The inhalation to the length of 12 digits of the Prana which has been exhaled, is called (the internal) Puraka (inhalation). This also is called the internal (Puraka), when Apana Vayu re-enters the body from outside without any effort. When Apana Vayu ceases to manifest itself and Prana gets absorbed in the heart, then the time occupied in such a state is (internal) Kumbha. Yogins are able to experience all these. When the Prana in the Akasa of the heart manifests itself externally (to the heart within) in diverse aspects without any affliction to the mind then it is called (the external) Rechaka (exhalation). When the externally fluctuating Prana enters the nose and stops there at its tip, then it is called the external Puraka. But when it is passing from the tip of the nose it goes down 12 digits. Then also it is called the external Puraka. When Prana goes arrested without and Apana within, then it is called the external Kumbhaka. When the shining Apana Vayu takes an upward bent within, then it is styled the external Rechaka. All these practices lead to Moksha. Therefore they should ever be meditated upon. Those who have understood and practised well all the external and internal Kumbhakas and others, will never be reborn.
"All the eight courses, I have given out before, are capable of yielding Moksha. They should be practised both day and night. Those who are associated with these practices smoothly, and control their minds by not letting them run in other directions, will in course of time attain Nirvana. Such practitioners will never thirst after material pleasures. They will ever be in their uniform practice, whether walking, standing, waking, dreaming or sleeping.
"Prana, having flown out, will again be absorbed in the heart having run back 12 digits. Similarly will Apana be absorbed in the heart, having issued out of the heart and running back 12 digits to it. Apana being the moon, will cool the whole body in its passage. But Prana being the sun, will generate heat in the system and cook or digest everything in it. Will pains arise in one who has reached that supreme state, where the Kalas (rays) of Apana the moon, are drowned by Prana the sun? Will rebirth arise in one who has reached that powerful seat, when the Kalas of Prana, the sun, are devoured by Apana the moon? These will arrest at once the seven births of those who reach that neutral state where they find Apana Vayu consumed by Prana and vice versa. I eulogie that Chidatma, who is in that intermediate state, where Prana and Apana are absorbed in one another. I meditate ceaselessly upon that Chidatma, who is in the Akasa, directly in front, at the end of my nose, where Prana and Apana both become extinct. Thus it is through this path of Prana's control, that I attained the supreme and immediate Tattva, devoid of pains."