Avidya is a Sanskrit word that means ignorance, misconceptions, misperceiving, illusion (Maya), misunderstandings, un-wisdom, or incorrect knowledge. It’s the opposite of the meaning of the word Vidya, which refers to true knowledge, true seeing, wisdom, and complete understanding.
The terms Avidya and Vidya are broadly used in Hinduism, Buddhism and Jainism, particularly in the context of knowing or not-knowing “the Spiritual Truth” or “the Real Self.”
In Advaita Vedanta, Avidya typically points to not-knowing our real identity, our True Self, the nature of Being, and what-the-world-is. This not-knowing is considered a form of spiritual ignorance (i.e. unawareness of what-is-the-case).
By contrast, when we firmly realize our True Nature — realize that Atman is Brahman, that We Are That — that then is Vidya, which entails subsequent Spiritual Awakening. Moreover, Advaita stresses that overcoming Avidya is mankind’s highest goal.
In a more general sense, one could say that Avidya is to confuse (or mistake) illusion (Maya) to be reality, or impermanence to be permanence, suffering being bliss, and non-self to be the Real Self.
Although Advaita Vedanta on a philosophical level states that the cause of Avidya (spiritual ignorance) is unknown (or even causeless), some Advaita teachers have claimed that it’s the Eternal Witness that identifies itself mistakenly with certain phenomena that appear in its own consciousness.
This then would be the cause of duality, by creating subject-object relationships, and “doers who do deeds.” This likewise creates the feeling (i.e. identification) that “I am this or that,” which should be negated by the Neti Neti approach of negation (I am not this, I am not that) if one aspires to attaining True Knowledge.
Nevertheless, why exactly Avidya occurs, remains vague as for the explanations, apart from that it’s just how we come into the world, that it has no cause, and that it’s part of the inherent dual constitution of consciousness.