Epiousios

Epiousios (Ancient Greek: επιούσιος) is a unique Greek word used in the fourth petition of the Lord's Prayer, as recorded in the Gospel of Matthew (Matthew 6:11) and the Gospel of Luke (Luke 11:3).

While often substituted by the word "daily," the Greek term otherwise used throughout the New Testament for "daily" is kath hemeran (καθ' ἡμέραν, "according to the day"). As no small challenge with respect to translation, the word epiousios is not found anywhere else in the original scriptures of the Bible, nor, moreover, anywhere else in all of Classical Greek literature.

Without reference except to the preceding words ("Give us today..."), Epiousios was rendered as quotidianum ("daily") in the Vetus Latina. However, more analytically, the word was also linguistically parsed to mean supersubstantialem ("super-substantial") in the Vulgate, albeit only in Matthew's version.

From the New American Bible Revised Edition:

Regardless of the lack of true correspondence of meaning, in the current era the word is nonetheless most often translated as "Give us this day our daily bread."