Prana is breath or respiration. Prana vayu is the air of the vital force, which pervades the entire human system. Ayama is the process of controlling it.

Tasmin sati svasa – prasvasayoh gati – vicchedah pranayamah
 
Pranayama is the regulation of inspiration and expiration of breath with retention.

Pranayama is the act of controlling the actions of breathing in and breathing out after establishing stability in an asana. Breathing in (inhalation) is called puraka and breathing out (exhalation) is called rechaka. Stopping both puraka and rechaka even for a little while is known as kumbhaka.

One of the great discoveries of yoga research is the connection between prana (breath) and the mind. Ancient yoga experts of India found that it was possible to establish control over the mind through the regulation of breathing.

Pranayama practitioners are taught to prolong the span of puraka, rechaka and even kumbhaka. Every twenty-four hours a person normally breathes 21,600 times (including inhalation and exhalation). Since the practice of pranayama extends the duration of breathing, it leads to longevity. The sadhaka, who has mastered pranayama, counts his span of life not in the number of days, months or years, but in the number of breaths.