A monstrance, also known as an ostensorium, is a vessel used in Roman Catholic, Old Catholic, High Church Lutheran and Anglican churches for the display on an altar of some object of piety, such as the consecrated Eucharistic Sacramental bread (host) during Eucharistic adoration or during the Benediction of the Blessed Sacrament. A monstrance may also serve 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 comes from the Latin word ostendere. Either term, each expressing the concept of "showing", can refer to a vessel intended for the exposition of the Blessed Sacrament, but ostensorium has only this meaning.

The Eucharist, also known as Holy Communion, Blessed Sacrament and the Lord's Supper, is a Christian rite that is considered a sacrament in most churches, and as an ordinance in others. Christians believe that the rite was instituted by Jesus at the Last Supper, the night before his crucifixion, giving his disciples bread and wine. Passages in the New Testament state that he commanded them to "do this in memory of me" while referring to the bread as "my body" and the cup of wine as "the blood of my covenant, which is poured out for many". According to the Synoptic Gospels this was at a Passover meal.