Yoga Sutras II.5  Anitya suci dukhanatmasu nitya suci sukhatma khyatir avidya.

Ignorance is the notion that takes the self, which is joyful, pure and eternal, to be the non-self, which is painful, unclean and temporary.

Anitya – not eternal, impermanent
asuci – impure
dukha – sorrow, grief, distress, pain
anatmasu – not spiritual, corporeal, something different from the soul
nitya –  eternal, everlasting, constant
suci –  pure
sukha –  joy, pleasure
atma –  soul
khyatih –  opinion, view, idea, assertion
avidya – ignorance, nescience

In this sutra, Patanjali defines what is meant by spiritual ignorance, which is the primary cause of suffering in human beings.  Avidya, ignorance, entails confusing the the nature of the soul with that of the body.  The body, or prakriti, is described here as painful, unclean and temporary.  The soul, Purusha, is defined as joyful, clean and eternal.  Patanjali is underscoring the fact that conventional awareness is the exact opposite of true knowledge.  To confuse the two is avidya.