Pranayama, in Sanskrit is composed of two words, 'prana' + 'ayama'. According to Yogic Philosophy it is understood that, Prana is the cosmic/universal life force or the vital energy which is responsible for keeping us 'alive'.
The word 'ayama' has two different meanings - to stretch/elongate/expand and to control/restrain.
Breath is a gross manifestation of this prana. So, the word pranayama means the ability to expand or stretch...