What is "improperia"

Improperia \Im`pro*pe"ri*a\, n. pl. [L., reproaches.] (Mus.) A series of antiphons and responses, expressing the sorrowful remonstrance of our Lord with his people; -- sung on the morning of the Good Friday in place of the usual daily Mass of the Roman ritual.
--Grove.

The Improperia are a series of antiphons and responses, expressing the remonstrance of Jesus Christ with His people. They are also known as the "Reproaches". In the Catholic liturgy they are sung as part of the observance of the Passion, usually on the afternoon of Good Friday. In the Byzantine Rite, they are found in various hymns of Good Friday and Holy Saturday. The Improperia appear in the Pontificale of Prudentius (846-61) and gradually came into use throughout Europe in the eleventh and twelfth centuries, finally being incorporated into the Roman Ordo in the fourteenth century.