Yoga Sūtra Chapter Two verse 5
अनित्याशुचिदुःखानात्मसु नित्यशुचिसुखात्मख्यातिरविद्या ॥५॥
anitya-aśuci-duḥkha-anātmasu nitya-śuci-sukha-ātma-khyātiḥ avidyā ||5||
Illusion is recognising what is transitory, impure, suffering
and other than essence, as eternal, pure, happiness and essence.
Commentaries and ReflectionsT Krishnamacharya:
When something is understood differently from what it truly is, it is called Avidyā.
What is changing is taken to be non-changing. For example the mind.
What is subjected to decay is assumed to be pure, for example the body.
What is leading to suffering is taken to be the source of pleasure.
What is not conscious is assumed to be conscious.
All these errors in perceptions have many possibilities.
But the ultimate stage of Avidyã is to assume that we are the Masters, not Īśvara.
”Avidyā is the illusion of recognising:
the ephemeral as the eternal,
the profane as the profound,
pain as pleasure and
the silhouette as the source.”