Avidyā. In Sanskrit, the lack of vidyā (true knowledge), i.e., ignorance. The upanishadic writers use the term, however, in a very complex way in their attempt to solve epistemological and metaphysical problems, including the virtually universal belief in spiritual evolution through a chain (saṃsāra) of embodiments governed by the karmic principle.

Geddes MacGregor, Dictionary of Religion and Philosophy, New York: Paragon House, 1989

avidyā

(Sanskrit; Pāli, avijjā). Ignorance; in Buddhism it refers specifically to ignorance about the workings of karma, the Four Noble Truths, and the Three Jewels (triratna). Avidyā is the root cause of continued involvement in saṃsāra and the experience of suffering by which one remains confused about the true nature of reality.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)

avidyā: Ignorance, nescience. It is not just the absence of vidyā, but a real existent functioning as positive force causing the non-cognizance. It operates in two modes: as saṃprayuktā avidyāand āveṇikī avidyā.

Sarvastivada Abhidharma, Sanskrit-English Glossary, by Bhikkhu KL Dhammajoti

【梵】avidyā
【滿】mentuhun
【蒙】munkhagan
【漢】無明

Pentaglot Dictionary of Buddhist Terms