Sutra 2.49 says: Tasmin satisvasa prasvasayoh gativicchedah pranayamah
sati – being accomplished
svasa – inbreath, inhalation
prasvasayoh – outbreath, exhalation
gati – movement, motion, path
vicchedah – cessation, stop
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pranayamah – 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.