​MASS, in music, a setting of certain portions of the mass to music; the portions of the mass usually set to music—namely, the Kyrie, the Gloria, the Credo, the Sanctus, and the Agnus Dei. An Offertory and Benedictus are sometimes added to these numbers. Masses are designated musically after the key in which they commence, as Beethoven in d; and liturgically, according to the character and solemnity of the accompanying ceremonial.