Pranayama is the discipline for establishing control over Prana in oneself. The most obvious movement of prana in the body is the motion of lungs in breathing. In breathing, it is not that breath is producing the motion of the lungs. It is the motion of lungs that helps the air to be drawn in by pump action. Pranayama is nothing but controlling that muscular power that moves the lungs. That power which goes out through the nerves to the lungs, moving them in a certain manner, is the Prana. When this expression of prana through breathing is controlled by an aspirant, all other actions of Prana in the body will slowly come under his control.
- Swami Vivekananda
(Excerpts from The Four Yogas of Swami Vivekananda, condensed and retold by Swami Tapasyananda)