An alpha privative or, rarely, privative a (from Latin alpha prīvātīvum, from Ancient Greek α στερητικόν) is the prefix a- or an- (before vowels) 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.

It is derived from a Proto-Indo-European syllabic nasal *n̥-, the zero ablaut grade of the negation *ne, i.e. /n/ used as a vowel. For this reason, it usually appears as an- before vowels (e.g. an-alphabetism, an-esthesia, an-archy). It shares the same root with the Greek prefix nē- or ne-, in Greek νη- or νε-, that is also privative (e.g. ne-penthe).

It is not to be confused with, among other things, an alpha copulative (e.g. a-delphi) or the prepositional component an- (i.e. the preposition ana with ecthlipsis or elision of its final vowel before a following vowel; e.g. an-ode).