From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) See Kyrie eleison.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

( Κύριε ), "O Lord" (in Church music), the vocative of the Greek word signifying Lord, with which word all the musical masses in the Church of Rome commence. Hence it has come to be used substantively for the whole piece, as one may say, A Beautiful Kyrie, A Kyrie Well Executed, etc.