A monstrance is the vessel used in the Roman Catholic, Old Catholic and Anglican churches to display the consecrated Eucharistic host, during Eucharistic adoration or Benediction of the Blessed Sacrament. Created in the medieval period for the public display of relics, the monstrance today is usually restricted for vessels used for Hosts. The word monstrance comes from the Latin word monstrare, meaning "to show", and is cognate with the English word demonstrate, meaning "to show clearly". In Latin, the monstrance is known as an ostensorium (from ostendere, "to show"). In Anglican churches, it is called a monstre/monstral.
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. Catholic doctrine holds that the elements are not only spiritually transformed, but are (substantially) transformed into the Body and Blood of Christ. The elements retain the appearance or "accidents" of bread and wine, but are the Body and Blood of Christ. This is what is meant by Real Presence; the presence of Jesus in the Eucharist. Because of this belief, the consecrated elements are given the same adoration and devotion that Christians accord to Christ.
Catholics believe that Christ is substantially present in the Eucharist, therefore 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.
The monstrance is usually elaborate in design; most are carried by the priest, if sometimes with some difficulty. Others may be much larger fixed constructions, typically for displaying the 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.
Medieval monstrances were more varied in form than contemporary ones; those for relics typically had a crystal cylinder in a golden stand, and those for Hosts had a crystal window in a flat-faced golden construction, which could stand on its base. The monstrance was most often made of silver-gilt or other precious metal, and highly decorated. 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 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. Before the current design, earlier "little shrines" or reliquaries of various shapes and sizes were used.
When the monstrance contains the Host, the priest will not touch the vessel with his bare hands. Out of respect, he holds it with a humeral veil, a wide band of cloth that covers his shoulders (humera) and has pleats on the inside, in which he places his hands. (Source: Wikipedia)