Epiousios (ἐπιούσιος) is a 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.
Since it is a Koine Greek dis legomenon found only in the New Testament passages Matthew 6:11 and Luke 11:3, 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 difficulty in understanding epiousios goes at least as far back as AD 382. At that time, St. Jerome was commissioned by Pope Damasus I to renew and consolidate the various collections of biblical texts in the Vetus Latina ("Old Latin") then in use by the Church. Jerome accomplished this by going back to the original Greek of the New Testament and translating it into Latin; his translation came to be known as the Vulgate. In the identical contexts of Matthew and Luke—that is, reporting the Lord's Prayer—Jerome translated epiousios in two different ways: by morphological analysis as 'supersubstantial' (supersubstantialem) in Matthew 6:11, but retaining 'daily' (quotidianum) in Luke 11:3.
The modern Catholic Catechism holds that there are several ways of understanding 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".:172