A Monstrance is the vessel used in the Roman Catholic, and Anglican Churches to display the consecrated Eucharistic Host, during Eucharistic adoration or Benediction of the Blessed Sacrament. The word monstrance comes from the Latin word monstrare, meaning "to show". It is closely related to the English word demonstrate, meaning "to show clearly".
In the Catholic tradition, at the moment of consecration the gifts are transformed into the actual Body and Blood of Christ. Catholic doctrine states that the elements are not only spiritually transformed, but are actually transformed into the Body and Blood of Christ. The elements retain the appearance or "accidents" of bread and wine, but are indeed the actual Body and Blood of Christ. This is what is meant by Real Presence, or the actual physical presence of Jesus in the Eucharist. Because of this belief, the concecrated gifts are given the same respect and adoration as we would give Christ (because it IS Christ). During Eucharistic adoration, the sacrament is displayed in the monstrance, most likely on the altar. When not being displayed, the 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 of 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.
The monstrance is usually very elaborate in design, with a sunburst design on a stand, usually topped by a cross. It is most often made of gold or other precious metal, and decorated with intricate sculpture work or other highly detailed designs. The monstrance is often made by hand by local or regional artists. In the center 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 luna, which holds the Host 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.
Whenever the monstrance contains the Host, the priest will not touch it with his bare hands, but instead out of respect holds it with a humeral veil, a wide band of cloth that covers his shoulders and has pleats on the inside in which he places his hands.