a-1 Before a vowel an-.
Not or without.
In more or less disguised form, this occurs in a number of words brought over from Greek, such as abyss (Greek bussos, depth); anomalous (Greek homalos, even); and anarchy (Greek arkhos, chief or ruler).
A variety of scientific and medical terms contain it, such as anaesthesic (US anesthetic)(Greek aisthēsis, sensation), something that prevents one feeling pain; amorphous (Greek morphē, form), without a clearly defined shape; and anorexia (Greek orexis, appetite), a medical condition in which there is a lack of appetite. Some have been created using native English stems: apolitical, not interested or involved in politics; atypical, not representative of a type or class.
A few can have slightly different senses to those in other negative prefixes on the same stem: someone who is immoral does not conform to moral precepts, but someone amoral lacks a moral sense; a person who is antisocial acts against the laws and customs of society, but one who is asocial is hostile to social interaction.
Despite these examples, the prefix is not often used to make new words.