The negative prefix a- or un- WordNet 3.0 & Open English WordNet

An alpha privative or, rarely, privative a is the prefix a- or an- that is used in Indo-European languages such as Sanskrit and Greek and in words borrowed therefrom to express negation or absence, for example the English words of greek origin atypical, anesthetic, and analgesic. Wikipedia

The 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. Wiktionary