A monstrance, also known as an ostensorium (or an ostensory), is a vessel used in Roman Catholic, Old Catholic, High Church Lutheran and Anglican churches for the more convenient exhibition of some object of piety, such as the consecrated Eucharistic host during Eucharistic adoration or Benediction of the Blessed Sacrament. It is also used as 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.
This is a photo from the perpetual adoration chapel at Saint Matthew's Church in Virginia Beach, Virginia.
This image is in the public domain.