What is Pranayama?

This is the definition of Pranayama in the Yoga-sutras of Patanjali.
‘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 hour 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 Vyashti, 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 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. 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.