Ignorance and the arising of dukkha Some explanations of the second Noble truth emphasizes the three poisons of ignorance, attachment and aversion as the causes of Dukkha. There are probably many different ways to lay this out, but my question is about ignorance, our misunderstanding of the nature of reality. 

What is the most basic and important deeply rooted ignorance about reality? 

What is it – on the most basic level – that we don’t see? 

What is it about the nature of reality that we are ignorant about? 

What is it that this bewilderment about reality consists of?
A definition of ignorance is as the first of the 'twelve nidānas'.
In the Paticca-samuppada-vibhanga Sutta it's described as follows:

And what is ignorance? Not knowing stress, not knowing the origination of stress, not knowing the cessation of stress, not knowing the way of practice leading to the cessation of stress: This is called ignorance.
Now from the remainderless fading &amp; cessation of that very ignorance comes the cessation of fabrications. etc.

So there it's defined as ignorance of the four noble truths.

This article titled Ignorance (by Thanissaro Bhikkhu) is a commentary about Ignorance from the perspective of the four noble truths (which is what you were asking about). It says that vijja (the opposite of avijjā i.e. ignorance) means not only 'knowledge' of facts but also 'skill'; so non-ignorance means:

Knowing the four noble truths
Understanding them
Skill, resulting in cessation and liberation


Avijja, the Pali word for ignorance, is the opposite of vijja, which means not only &quot;knowledge&quot; but also &quot;skill&quot; — as in the skills of a doctor or animal-trainer. So when the Buddha focuses on the ignorance that causes stress and suffering, saying that people suffer from not knowing the four noble truths, he's not simply saying that they lack information or direct knowledge of those truths. He's also saying that they lack skill in handling them. They suffer because they don't know what they're doing.


The Sammaditthi Sutta concurs in equating ignorance with the four noble truths ... it says that not knowing about the four noble truths is the origin of ignorance:

And what is ignorance, what is the origin of ignorance, what is the cessation of ignorance, what is the way leading to the cessation of ignorance? Not knowing about suffering, not knowing about the origin of suffering, not knowing about the cessation of suffering, not knowing about the way leading to the cessation of suffering — this is called ignorance. With the arising of the taints there is the arising of ignorance. With the cessation of the taints there is the cessation of ignorance. The way leading to the cessation of ignorance is just this Noble Eightfold Path; that is, right view... right concentration.
When a noble disciple has thus understood ignorance, the origin of ignorance, the cessation of ignorance, and the way leading to the cessation of ignorance... he here and now makes an end of suffering. In that way too a noble disciple is one of right view... and has arrived at this true Dhamma.

If cessation of ignorance is right view, then presumably &quot;ignorance&quot; can be defined as or understood as the cause of 'wrong view'.
The Maha-satipatthana Sutta too defines 'right view' as knowledge of the four noble truths.

And what is right view? Knowledge with regard to stress, knowledge with regard to the origination of stress, knowledge with regard to the cessation of stress, knowledge with regard to the way of practice leading to the cessation of stress: This is called right view.


There's a (closely) related term moha which is one of the three poisons and one of the defilements (I think that identifying three poisons is more Mahayana than Theravada, which sees aversion as a secondary cause, as in Sankha's answer).
When it's ignorance or illusion or delusion about the three characteristics, I think that means for example:

Seeing the non-self as self
Seeing the impermanent as permanent
Seeing the inherently-unsatisfactory as desirable.


Ignorance (like conceit) is one of the last of the fetters to be eradicated, according to the stages of the path (though identity-view is a wrong view and therefore one form of ignorance).