The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]