improperia

from The Century Dictionary.

• Antiphons and responses which on Good Friday are substituted for the usual mass of the Roman ritual.

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

• noun plural (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.