Yoga is the stilling of the mind
Yoga Sūtras of Patañjali · 1.2
The Yoga Sūtras' second sūtra, the whole system in four words.
yogaś citta-vṛtti-nirodhaḥ ||
Yoga is the stilling (nirodha) of the turnings (vṛtti) of the mind-field (citta).
Patañjali defines yoga not as posture, breath or even meditation, but as the condition when the mind’s turning (vṛtti) stills. The next sūtra adds the reason it matters: “then the seer rests in its own nature.” Everything else in the 195 sūtras, including the eight limbs; is scaffolding for this one definition.
This site’s whole meditation page is downstream of this sentence; the soul-pillar’s ashtanga pages map the scaffolding. Four words, and a psychology two millennia old that modern attention research keeps arriving beside.