Kyrie eleison

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The Kyrie.

from The Century Dictionary.

• noun Literally, Lord, have mercy! a brief petition, founded on nearly identical Scriptural phrases (for example, Ps. cxxiii. 3, Mat. xx. 30), used as a response in the primitive liturgies and in the eucharistic and other offices of Oriental churches to the present day.
• noun The first movement or division in a musical setting of a Roman Catholic mass or the Anglican communion office, the text being the petitions above mentioned.

from the GNU version of the Collaborative International Dictionary of English.

• noun (R. C. Ch.) Greek words, meaning “Lord, have mercy upon us,” used in the Mass, the breviary offices, the litany of the saints, etc.
• noun The name given to the response to the Commandments, in the service of the Church of England and of the Protestant Episcopal Church.