1.NounThe prefix a- (or an- when prefixing a root which begins with a vowel) found in some English words of Greek derivation. It expresses negation or absence. The prefix has cognates in other Indo-European languages, including in- in Latin and un- in English, all traceable back to Proto-Indo-European *n̥-.
2.NounEither the aforementioned privative a or the Greek ἀ- (a-) itself (from which the English descendant derives).