From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) The psalm usually appointed for penitential acts, being the 50th psalm in the Latin version. It commences with the word miserere.

(2): ( n.) A musical composition adapted to the 50th psalm.

(3): ( n.) A small projecting boss or bracket, on the under side of the hinged seat of a church stall (see Stall). It was intended, the seat being turned up, to give some support to a worshiper when standing. Called also misericordia.

(4): ( n.) Same as Ileus.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

(Lat. have compassion), the name of a liturgic prayer, set to music, and used in Roman Catholic worship. It is a sort of paraphrase on the 51st or 57th Psalm, and is used on penitential occasions, and particularly in Passion-week. It is therefore not only set to a regular Gregorian melody (see Keller, Die acht Psalmentone, etc., Aix-la-Chap. 1856, page 18), but has also become a theme for compositions to the most eminent masters, such as Palestrina, Orlando di Lasso, Allegri, Scarlatti, Leonardo Leo, Thomas Bai, Zingarelli, Pergolese, Jomelli, Fioravanti, Fdtis, Vogler, Stadler, etc. The most renowned among these compositions is that by Gregorio Allegri (a descendant of Correggio, born at Rome in 1590, t 1640), in which two choirs, one of four, the other of five parts, sing alternately until the finale, where all join in pianissimo, the measure also becoming gradually slower. This piece, from the time it was composed, has always been sung on Wednesday and Friday of Passion-week in the Sistine Chapel at Rome. One writer says: "Never by mortal ear was heard a strain of such powerful, such heart-moving pathos. The accordant tones of a hundred human voices, and one which seemed more than human, ascended together to heaven for mercy to mankind for pardon to a guilty and sinning world. It had nothing in it of this earth nothing that breathed the ordinary feelings of our nature. Its effects upon the minds of those who heard it were almost too powerful to be borne, and never can be forgotten. One gentleman fainted and was carried out; and many of the ladies near me were in agitation even more distressing, which they vainly struggled to suppress. It was the music of Allegri; but the composition, however fine, is nothing without the voices which perform it here." Another writer says: "At the conclusion of this portion of the service, and when the darkness is complete by the concealment of the last light, commences the Miserere. This is the 51st Psalm. And as it is breathed by the choir the most perfect and practiced choir in the world as it is heard in all the stillness and solemnity of the scene, wrapped in darkness, and leaving nothing to distract the eye where all looks dim and shadowy, it has a strange and wonderful effect. It is designed to express, as far as music can express, the deep and mental agonies of the dying Saviour; and certainly there never yet was heard, except among the shepherds of Bethlehem on the night of the nativity, such sounds, so unearthly, and unlike the music of the world. It is plaintive, intensely melancholy, and has a powerful effect under the peculiar circumstances of the scene." It was formerly the exclusive property of the Sistine Chapel, the partition being jealously kept there; Mozart succeeded, however, in writing it down after hearing it twice. It has since been repeatedly published. While the Miserere is sung, the pope kneels at the altar, the cardinals at their desks, and as it proceeds the lights at the altar are extinguished one by one, which is explained by Gavanti, Thes. 2:99: "Ad unumquemque psalmum (there are other psalms sung before the Miserere) exstinguitur una candela, una post aliam, quia apostoli paulatim defecerunt a Christo." In fact, the whole use of this psalm in Passion-week is intended ad designandum apostolorumn timorem. The word miserere has in modern days come to be applied to any sacred composition of a penitential character. See Herzog, Real-Encyklopadie, 9:547; Eadie, Eccles. Cyclop. s.v.; Siegel, Christliche Alterthumer (see Index in volume 4).

The Nuttall Encyclopedia [3]

A carved bracket on the under side of the stall seats in mediæval churches, which, when the seat was turned up during the standing portion of the service, afforded support to the older clergy. Miserere, the Catholic name for the 51st Psalm.