’Yoga Pradipika’, Pranayama is the fourth limb of Ashtanga Yoga 

He defines pranayama as ‘the regulation of the movements of inhalation and exhalation’. 

Prana’ is the vital life force that acts as a catalyst in all our activities  

and ‘ayama’ refers to the expansion or control of this vital force 

‘Prana’ also means breath, respiration, life, vitality, wind, energy or strength  

while ’ayama’ also means length, expansion, stretching and restraint. 

In short, pranayama is the 'science of breath’ and is the hub around which the wheel of life revolves. 

It is regulated by place, time and number 

This means that at various times in our Yoga Sadhana, different Pranayamas