"Pranayama is control of Breath". "Prana" is Breath or vital energy in the body. On subtle levels prana represents the pranic energy responsible for life or life force, and "ayama" means control. So Pranayama is "Control of Breath"
Pranayama is the perfect control of the life-currents through control of breath, and is the process by which we understand the secret of prana and manipulate it. You can hardly make any spiritual progress without the practice of pranayama. One who has grasped this prana has grasped the very core of cosmic life and activity. Through various exercises and training in pranayama the yogi tries to realize in this little body the whole of cosmic life, and attain perfection.
Breath is a physical aspect or external manifestation of prana, the vital force, and thus pranayama begins with the regulation of the breath. Breath, like electricity, is gross prana, while prana itself is subtle. By controlling the breath you can control the prana – just as you can control the other wheels by controlling or stopping the fly wheel of a diesel engine, and just as you can control the hairspring, cog wheels and the main spring of a watch by controlling the minute hand. Control of breath is achieved through manipulation of the lungs and the breathing process.