Either simply not knowing or knowing in an inverted, incorrect manner either (1) behavioral cause and effect, or (2) voidness (emptiness), the total absence of impossible ways of existing. Translators often render the term as "ignorance."
(1) According to Vasubandhu and Asanga, not knowing. (2) According to Dharmakirti, knowing in an inverted (incorrect) manner. In both cases, unawareness is of either behavioral cause and effect, or of the lack of an impossible "soul" or voidness. Translators often render the term as "ignorance."
J Hopkins: ignorance.