Sutra 2.49 says: Tasmin satisvasa prasvasayoh gativicchedah pranayamah

Tasmin -  on this

sati – being accomplished

svasa – inbreath, inhalation

prasvasayoh – outbreath, exhalation

gati – movement, motion, path

vicchedah – cessation, stop

pagepranayamah – ­regulation.

Pranayama is the regulation of the incoming and outgoing flow of breath with retention.  It is to be practiced only after perfection in asana is achieved. It's more then just breathing friends.