The Monstrance, also known as an ostensorium (or an ostensory), is the vessel used in the Roman Catholic Church to exhibit objects of devotion, such as the Consecrated Eucharistic Host during Eucharistic Adoration or Benediction of the Blessed Sacrament. It is also used as a reliquary for the public display of relics of some saints. The word Monstrance comes from the Latin word monstrare, while the word ostensorium came from the Latin word ostendere. Both terms, meaning “to show”, are used for vessels intended for the exposition of the Blessed Sacrament, but the ostensorium has only this meaning.
In the Catholic Church Tradition, the reserved Sacrament serves as a focal point of religious devotion. During Eucharistic Adoration, the celebrant displays the Sacrament in the Monstrance, typically on the Altar. When not being displayed, the reserved Sacrament is locked in the 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. The exposition of the Monstrance during Benediction is traditionally accompanied by chanting or singing of the hymn Tantum Ergo.
Monstrances are usually elaborate in design; most are carried by the priest. Others may be much larger fixed constructions, typically for displaying the Consecrated Host in a special side 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.”