Epiousion (ἐπιούσιον) is a Koine Greek adjective used in the Lord's Prayer verse "Τὸν ἄρτον ἡμῶν τὸν ἐπιούσιον δὸς ἡμῖν σήμερον" ('Give us today our epiousion bread'). Because the word is used nowhere else, its meaning is unclear. It is traditionally translated as "daily", but most modern scholars reject that interpretation. The word is also referred to by epiousios, its presumed lemma form.

Since it is a Koine Greek tris legomenon (a word that occurs only thrice within a given corpus) found only in the New Testament passages Matthew 6:11 and Luke 11:3 and in the early Christian treatise Didache, but always in the same context of relaying the Lord's Prayer, its interpretation relies upon morphological analysis and context. The traditional and most common English translation is daily, although most scholars today reject this in part because all other New Testament passages with the translation "daily" include the word hemera (ἡμέρᾱ, 'day').

The Catechism of the Catholic Church holds that there are several ways of understanding epiousion (which the Catechism calls epiousios), including the traditional 'daily', but most literally as 'supersubstantial' or 'superessential', based on its morphological components. Alternative theories are that—aside from the etymology of ousia, meaning 'substance'—it may be derived from either of the verbs einai (εἶναι), meaning "to be", or ienai (ἰέναι), meaning both "to come" and "to go".

A majority of scholars today believe that epiousion probably meant "for tomorrow" or "for the future".

The word is visible in the Hanna Papyrus 1 (𝔓⁷⁵), the oldest surviving witness for certain New Testament passages.

Epiousion is the only adjective in the Lord's Prayer. It is masculine, accusative, singular, agreeing in gender, number, and case with the noun it qualifies, ἄρτον, arton ("bread"). In an interlinear gloss:

Τὸν

The