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, since the mind will always move. It is identification with the movement, mistaking the ripple for yourself, that practice slowly loosens.
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.