A "monstrance" is a vessel used in the Roman Catholic Church to display a sacred object, such as the consecrated Host or relics from the Saints. It is also called an ostensorium.
The monstrance is typically transparent or open so that it is easy to view the Host. It generally has an elaborate design and is carried by the priest. The priest blesses the congregation with the monstrance during the service of Benediction, after the Host has been consecrated.
The word "monstrance" was first used to describe this vessel in English between 1250-1300. It comes from medieval Latin "monstrantia," which derives from the Latin "monstrare," meaning "to show."