Short answers, pulled from the story.
Pranayama is a Sanskrit compound combining prana, meaning breath or life energy, with a second element interpreted variously as suspension, control, or expansion. Scholars including Macdonell, Monier-Williams, V. S. Apte, and Ramamurti Mishra each give different definitions, ranging from the suspension of breath to the expansion of individual energy into cosmic energy.
Pranayama appears in verse 4.29 of the Bhagavad Gita, which describes practitioners who stop all breathing as a form of sacrifice. Patanjali addresses it as the fourth of eight limbs of yoga in the Yoga Sutras, discussing it in verses 2.49 through 2.53 and introducing it as a method to stabilize the mind in verse 1.34.
The 15th century Hatha Yoga Pradipika describes several named techniques, including Kumbhaka (breath retention), Nadi Shodhana (alternate nostril breathing), Bhastrika (bellows breath), Kapalabhati (skull-shining breath), Ujjayi (victorious breath), Surya Bhedana (sun-piercing breath), Bhramari (humming breath), and Sitali (breathing through a rolled tongue).
Swami Kuvalayananda began the experimental study of pranayama in 1924. From 1929 he conducted studies on pranayama's effects on the respiratory system, including a 1956 study with Karambelkar on urine acidity following prolonged bhastrika pranayama. A 2018 systematic review by Andrea Zaccaro and colleagues surveyed 2,461 research articles, finding only 15 that met strict eligibility criteria.
In classical texts such as the Bhagavad Gita and the Yoga Sutras, pranayama referred to the complete cessation of breathing. In modern vinyasa yoga, as taught by Sharath Jois, pranayama means coordinating the breath with movement between postures. The yoga scholar Andrea Jain describes these breathing practices as dramatically unlike one another.
Before his enlightenment, the Buddha practiced a technique involving pressing the palate with the tongue and forcibly restraining the breath, described in the Pali Buddhist Canon as both extremely painful and not conducive to enlightenment. His later teaching in the Anapanasati Sutta used moderate breath modulation as preparation for concentration rather than forcible suspension.