Prana
prāṇaLife force, or simply breath-energy: the currency of the hatha tradition.
The Hatha Yoga Pradipika defines prana with startling directness, as the difference between a living body and a dead one:
Breath is treated as life itself: while the air stays in the body there is life, and death is its leaving. Training the breath is therefore the central discipline, not an accessory.
“So long as the (breathing) air stays in the body, it is called life. Death consists in the passing out of the (breathing) air. It is, therefore, necessary to restrain the breath.”
The original Sanskrit
yāvad vāyuḥ sthito dehe tāvaj jīvanam ucyate / maraṇaṃ tasya niṣkrāntis tato vāyuṃ nirodhayet
Breath is life; its restraint is the practice. Everything else in the hatha toolbox, poses, locks, channels, exists to work on this.