p. 246
निष्कल्मषं तपस्त्वेतत्केवलं परिचक्षते ।
In plain words
Austerity with no stain in it, nothing mixed in — that, and only that, is what they call tapas.
Close to the Sanskrit
But this stainless, unmixed austerity — this alone they declare to be austerity.