Epiousios (Greek: Επιούσιος) is a Greek word used in the fourth petition of the Lord's Prayer, as it is recorded in the Gospel of Matthew and in the Gospel of Luke. In English epiousios is usually translated as "daily", as in "Give us this day our daily bread".
Epiousios has no direct or simple English translation (although see super-essential below) and there have been several interpretations of its meaning throughout the history of Christianity. For Christians, this is not just a quibble over an isolated phrase. Christians believe that the Lord's Prayer was instituted by Jesus for the use of his disciples, so they want to be as faithful as they can be to the original words of Jesus. Beyond that, subtle differences among various translations of this prayer become larger differences when the various translations are used to generate theology. The words Jesus used to teach his followers to pray reveals something of how he viewed himself, God, and the earthly life of his followers.
Epiousios is an example of how the translation of one word can make a significant theological difference: if the phrase "ton arton hēmōn ton epiousion" (τὸν ἄρτον ἡμῶν τὸν ἐπιούσιον ) is translated into English as "our daily bread", this imparts to the reader that Jesus wanted his followers to ask God for the means to survive physically, one day at a time. If, on the other hand, it is translated as "our bread for tomorrow", Jesus is saying that we should pray for our future needs rather than our present needs. A third possibility is "our necessary" or "our essential bread".
The foregoing translations imply ordinary bread that we eat every day to sustain our bodies; a fourth possibility, "our bread for the age to come", implies a spiritual bread or nourishment. Still other translations would focus attention beyond ordinary bread and onto the Eucharist: epiousios has also been translated as supersubstantial or as something having to do with the very essence of things rather than their tangible nature, or as "supernatural".
It has been proposed that "ho artos hēmōn ho epiousios", whatever it may mean, was used as a name for the Eucharist by the earliest Christians, even before the Gospel accounts of Jesus's life were written. If we take the sacramental use of this phrase as being its primary meaning, this would indicate that the Gospel authors used epiousios with a specific meaning in mind: "our eucharistic bread"; however, there is too little actual evidence to indicate that the sacramental meaning of the phrase was in fact its seminal use, and this was not the only phrase used to describe the eucharistic feast.
A common way to infer the unknown meaning of an ancient word is to look at all of the various contexts in which that word is used in ancient writings. For epiousios, however, this method is difficult to apply, because the word is found hardly anywhere [in fact nowhere] else in Greek or Hellenistic literature. Its use was long thought to be restricted to the two versions of the Lord's Prayer in Matthew and Luke. This would have made it a hapax legomenon (a Greek phrase meaning 'a word used only once') - a word used only in Christian circles and lacking meaning outside of a eucharistic context.
It was only in the twentieth century that a single additional use of the word seemed to be discovered. The document in which it was found is a 5th century CE shopping list (identified as SB1,5224 = Flinders Petrie Hawara p. 34). The word epiousios is written next to the names of several grocery items. This seems to indicate that it was used in the sense of "enough for today", "enough for tomorrow", or "necessary". However, the papyrus containing the shopping list went missing for many years, until it was discovered in 1998 at the Yale Beinecke Library. The original transcriber, one A. H. Sayce, was apparently known as a poor transcriber, and re-examination of the papyrus found "elaiou" (oil) but not epiousi.... So there seems indeed to be no other occurrence of the word in Greek literature.
A Greek-English Lexicon of the New Testament and Other Early Christian Literature, edited by Bauer, Arndt, Gingrich, Danker, University of Chicago Press, the standard lexicon for NT Greek, while noting historical interpretations and modern opinions, concludes that Origen was probably correct that the term was coined by the evangelists (Danker, the current editor, was familiar with the papyrus history above). It lists four possible translations: 1. deriving from Epi and Ousia: necessary for existence, in agreement with Origen, Chrysostom , Jerome and others; 2. one loaf of bread is the daily requirement; 3. for the following day; 4. deriving from epienai: bread for the future. In Jerome's translation, made in 405 A.D. we read (Mat 6:11): "Give us this day our supersubstantial bread" ("panem nostrum supersubstantialem da nobis hodie").
The word or prefix epi occurs over 300 times in the Gospels. Most often it means above, over, on, upon, besides, or in addition to. In a number of contexts it is translated into Latin as super. For example where epi appears in the Greek NT we read in the Vulgate: (Mt 14:25) 'ambulans super mare' 'walking upon the sea'; and (Mt 18:13) 'quia gaudet super eam magis quam super nonaginta novem' 'he rejoiceth more over that, than over the ninety-nine'; and Lk 1:35 'Spiritus Sanctus superveniet te' 'The Holy Spirit will come upon thee'.
'Ousios [ousia] means being, substance, essence or nature. The Council of Nicea in 325 AD used the term homoousios - from homos, same, and ousia, essence - to mean of one essence or substance. Hence homoousios in Latin is consubstantialem.
According to the Catechism of the Catholic Church, the literal translation of epiousios is super-substantiale  or in English, super-essential.. In 1551 the Council of Trent described the Holy Eucharist as supersubstantial bread.
Bauer, Arndt, Gingrich, Danker, eds. A Greek-English Lexicon of the New Testament and Other Early Christian Literature. University of Chicago Press (the Bauer lexicon).
M. Nijman and K. A. Worp. "ΕΠΙΟΥΣΙΟΣ in a documentary papyrus?". Novum Testamentum XLI (1999) 3 (July), p. 231-234.
F. Preisigke, Sammelbuch griechischer Urkunden aus Ägypten 1.5224:20.
B.M. Metzger, "How Many Times Does ΕΠΙΟΥΣΙΟΣ Occur outside The Lord's Prayer?" ExpTimes 69 (1957-58) 52-54.