The Monstrance also was known as the Ostensorium in accordance with its etymology. It is the vessel used in the Roman Catholic Church for the more convenient exhibition of some object of piety. Such as the consecrated Eucharistic host during Eucharistic adoration or Benediction of the Blessed Sacrament. This word monstrance comes from the Latin word “monstrare” while the word ostensorium came from the Latin word “ostendere”.
Both terms, mean “to show”, are used for vessels intended for the exposition of the Blessed Sacrament, but ostensorium has only this meaning.[In the Catholic tradition, at the moment of consecration the elements (called “gifts” for liturgical purposes) are transformed (literally transubstantiated) into the body and blood of Christ. Both the name ostensorium and the kindred word monstrance were originally referred to all kinds of vessels of goldsmith’s or silversmith’s work in which glass, crystal, and so on were so used to distinguish contents from another – Whether the object thus honoured were the Sacred Host itself or only the relic of some saint. Modern times has limited both terms to vessels intended for the exposition of the Blessed Sacrament, and it is in this sense only that we use ostensorium/Monstrance.
According to the Catholic doctrine, the elements are not only spiritually transformed but are (substantially) transformed into the body and blood of Christ. Although the elements retain the appearance, or “accidents,” of bread and wine, they become the body and blood of Christ, and the presence of Jesus in the Eucharist is known as the doctrine of the Corporeal Presence within the Roman Catholic tradition. During the Eucharistic adoration, the celebrant displays the sacrament in the monstrance, typically on the altar. The exposition of the monstrance during Benediction is traditionally accompanied by chanting or singing of the hymn Tantum Ergo.
When not being displayed, the reserved sacrament is locked in a tabernacle In the service of Benediction, the priest blesses the people with the Eucharist displayed in the monstrance. This blessing differs from the priest’s blessing, as it is seen to be the blessing by Christ rather than that of the individual priest.
Monstrances are usually different in design. Most of them are carried by the priest. Others may be fixed at a place, typically for displaying the host in a special side or a chapel, often called the “Chapel of the Blessed Sacrament”. For portable designs, the preferred form is a sunburst on a stand, usually topped by a cross.
During Medieval times, monstrances were more varied in form than contemporary ones. Those used for relics, and occasionally for the host, typically had a crystal cylinder in a golden stand, while those usually used for hosts had a crystal window in a flat-faced golden construction, which could stand on its base.
In the centre of the sunburst, the monstrance normally has a small round glass the size of a Host, through which the Blessed Sacrament can be seen. Behind this glass is a round container made of glass and gilded metal, called a lunette. This holds the Host securely in place. When not in the monstrance, the Host in its luna is placed in a special standing container, called a standing pyx, in the Tabernacle.