In the Eastern Christianity (including be Eastern Orthodox, Oriental Orthodox and Eastern Catholic), the phrase Kýrie, eléison (Greek: Κύριε ἐλέησον) or its equivalents in other languages is the most oft-repeated phrase[citation needed].

The various litanies, popular in Orthodox Christianity, generally have Lord, have mercy as their response, either singly or triply. Some petitions in these litanies will have twelve or even forty repetitions of the phrase as a response.

The phrase is the origin of the Jesus Prayer, beloved of Eastern Christians belonging to the Byzantine rite, and increasingly popular amongst Western Christians today.

The biblical roots of this prayer first appear in 1 Chronicles 16:34

...give thanks unto the LORD; for he is good; for his mercy endureth for ever...

This is key to fully understanding the Greek Kýrie, eléison. In this respect, the prayer is simultaneously a petition and a prayer of thanksgiving; an acknowledgment of what God has done, what God is doing, and what God will continue to do. This prayer is refined by Jesus in Luke 18:9-14 (KJV) The Parable of The Publican, where we see more clearly the connection to the Jesus Prayer: "God, have mercy on me, a sinner!" (KJV)

The Mass/Divine Liturgy was first celebrated in Greek at Rome during the first two centuries of The Church. As Latin became the predominant language, The Mass was translated into Latin. However, the familiar and venerated prayer Kýrie, eléison was later inserted back into The Mass, replacing the Latin "Domine, Miserere!"

The Greek phrase Kýrie, eléison has also been regularly and extensively used in Coptic (Egyptian) Christian churches since the early centuries of Christianity, where in liturgy both Coptic and Greek languages are used. The Coptic and Greek languages share many letters, words, and phrases, particularly in ecclesiastical contexts.

The Kyrie prayer, offered during the Roman Catholic Mass and in some other denominations (such as Lutheran and many in the Anglican Communion), led by the priest or celebrant, and repeated by the congregation. It is conjectured by scholars, including Jungmann, that the Kyrie in the Roman Mass is a vestigial remnant of a litany at the beginning of the mass, much like that of the Eastern Churches. Though today usually recited in the vernacular, the traditional form of the Kyrie in Western Christianity is a transliteration of the Greek prayer into Latin, and is used in this form in Latin-language Masses.

Traditionally, each line was sung three times. The three lines being sung thrice is an allusion to the Trinity.