Patañjali's Yoga Sūtra 1.2 is three Sanskrit words:
योगश्चित्तवृत्तिनिरोधः
yogaḥ citta-vṛtti-nirodhaḥ
The English translation industry has produced perhaps a hundred versions. "Yoga is the cessation of the modifications of the mind." "Yoga is the stilling of the changing states of the mind." "Yoga is the restraint of mental fluctuations." "Yoga is the absence of fluctuations in consciousness."
Each captures something. Each loses something. Let us go slowly.
Yoga (योग). From the root yuj, "to yoke". The same root as English yoke, conjugal, junction. The semantic range covers union, restraint, discipline, and method. Patañjali is not using it loosely. In this opening, yoga is being defined. Whatever yoga turns out to be, it is what this sūtra is about to say.
Citta (चित्त). The mind—but not exactly. Citta is the whole psychic apparatus: intellect (buddhi), ego-sense (ahaṃkāra), and the lower mind (manas) considered together. It is what does the seeing, the wanting, the remembering. Not the brain. The functional totality of the inner instrument.
Vṛtti (वृत्ति). From the root vṛt, "to turn". A turning, a modification, a wave. The mind is not still water; it is constantly modifying its shape according to its objects. Patañjali lists five kinds in Sūtra 1.6: right knowledge, wrong knowledge, imagination, sleep, and memory. Even sleep is a vṛtti. The mind in sleep is not absent; it is taking the shape of unconsciousness.
Nirodha (निरोध). The hard word. Usually translated "cessation" or "restraint", both of which mislead. The root rudh means "to obstruct", "to enclose", but with the prefix ni- it takes a more specific sense: containment, holding back, the deliberate non-movement of something that would otherwise move.
Nirodha is not killing the mind. It is the mind not turning. The water that could ripple but does not. The energy is there. The motion is not.
The classical commentator Vyāsa, writing perhaps four hundred years after Patañjali, glosses nirodha as vṛtti-abhāva—the absence of vṛtti. Not their suppression. Their absence.
Putting it together.
The sūtra is saying: yoga is what happens when the psychic apparatus stops turning. The technical content is contained in the genitive compound. Citta is the agent of vṛtti. Vṛtti is the activity of citta. Nirodha is what arrives when that activity ceases.
This is not a state of unconsciousness. The next sūtra (1.3) makes this explicit: tadā draṣṭuḥ svarūpe avasthānam—"then the seer abides in its own nature". When the modifications cease, the seer (the witnessing consciousness) is left, simply, as itself.
The whole of Patañjali's system follows from these three words. The eight limbs, the moral preparations, the postures, the breath disciplines, the meditative absorptions—all of it is in service of producing, at last, the state described in 1.2.
The sūtra is approximately two thousand years old. It is approximately the right size.