Yoga's whole aim is chitta vritti nirodhah, the stilling of the fluctuations of the mind. Two terms make that practical.
The mind-field (Chitta) is the whole field of mind-stuff: consciousness, memory, impressions, and everything that arises within them. Not just "thoughts," but the entire mental substrate, the surface on which experience appears.
A fluctuation (Vritti) is any movement of that field. The tradition counts five kinds: right knowledge (pramana), misconception (viparyaya), imagination (vikalpa), sleep (nidra), and memory (smriti). The problem is never the vritti itself. Movement is what a mind does, and the stilling Patanjali names is the far end of a long road. What practice loosens first, and can loosen today, is identification with the movement, mistaking the ripple for yourself.
The five, each with the picture we use for it:
- Right knowledge (pramana) — a ruler. Seeing, reasoning and a trustworthy report measure the world as it is. Even a true thought is still a movement of the mind.
- Misconception (viparyaya) — a straw in a glass of water. The eye sees it bend at the surface, and nothing is bent. The error is in the seeing, not in the thing.
- Imagination (vikalpa) — a thought bubble. Words and pictures with nothing behind them, which is most of what a mind does in a day.
- Sleep (nidra) — a pillow. Even deep sleep counts as a fluctuation: you wake knowing you slept well, so something was there to notice it.
- Memory (smriti) — a footprint. What was walked leaves a trace, and the trace outlasts the walk. The mind keeps stepping back into it.
Picture a lake: disturbed, its surface distorts what it reflects; still, it shows things as they are. The whole teaching opens the Yoga Philosophy page.