The privative a (or a privativum) is the prefix a- expressing negation (e.g. a-theism, a-neurism). Originally described for the grammar of Ancient Greek, it goes back to a PIE syllabic nasal *n̥-, the zero ablaut grade of the negation *ne. For this reason, it appears as an- before vowel (e.g. an-alphabetism). The same prefix appears in Sanskrit, also as a-, an-. In Latin, the cognate prefix is in-, and in West Germanic languages (including English) it is un-. In North Germanic languages, the -n- has disappeared and Swedish has o-, whereas Danish and Norwegian have u-.
See also: copulative a