“Pranayama is control of Breath”. “Prana” is a 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.