The definition

“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 dukkha, not knowing about the origin of dukkha, not knowing about the cessation of dukkha, not knowing about the way leading to the cessation of dukkha — 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.”

…

“And what are the taints, what is the origin of the taints, what is the cessation of the taints, what is the way leading to the cessation of the taints? There are three taints: the taint of sensual desire, the taint of being and the taint of ignorance. With the arising of ignorance there is the arising of the taints. With the cessation of ignorance there is the cessation of the taints. The way leading to the cessation of the taints is just this Noble Eightfold Path; that is, right view… right concentration.”

— MN 9 (Ñanamoli/Bodhi, trans.)

Because ignorance is the root cause of dukkha…

“From ignorance as a requisite condition come fabrications. From fabrications… comes consciousness. From consciousness… name-&-form. From name-&-form… the six sense media. From the six sense media… contact. From contact… feeling. From feeling…craving. From craving… clinging/sustenance. From clinging/sustenance… becoming. From becoming… birth. From birth as a requisite condition, then aging & death, sorrow, lamentation, pain, distress, & despair come into play. Such is the origination of this entire mass of stress & suffering.

…when it ceases, so too must dukkha cease

“Now from the remainderless fading & cessation of that very ignorance comes the cessation of fabrications. From the cessation of fabrications comes the cessation of consciousness. From the cessation of consciousness comes the cessation of name-&-form. From the cessation of name-&-form comes the cessation of the six sense media. From the cessation of the six sense media comes the cessation of contact. From the cessation of contact comes the cessation of feeling. From the cessation of feeling comes the cessation of craving. From the cessation of craving comes the cessation of clinging/sustenance. From the cessation of clinging/sustenance comes the cessation of becoming. From the cessation of becoming comes the cessation of birth. From the cessation of birth, then aging & death, sorrow, lamentation, pain, distress, & despair all cease. Such is the cessation of this entire mass of stress & suffering.”

— SN 12.2

See also:

·         The Four Noble Truths

·         The Second Noble Truth