'apeiron' originates from Ancient Greek, specifically the word 'ἄπειρον' (ápeiron), where the prefix 'a-' meant 'not' and 'peiron' meant 'limit' or 'boundary'.

'ἄπειρον' passed into scholarly Latin and later modern languages largely by direct transliteration and was adopted into English as the technical philosophical term 'apeiron' in discussions of pre-Socratic thought.

Initially it meant 'without limit' or 'boundless' in Ancient Greek; over time it became a technical philosophical term in English denoting the indefinite or unlimited principle (especially in Anaximander's philosophy).