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.

The lunette, known in Germany as the lunula and also as the melchisedech, is a crescent-shaped clip made of gold or of silver-gilt which is used for holding the Host in an upright position when exposed in the monstrance. The crescent which holds the Host is securely attached to a small stand or frame and the receptacle of the monstrance is usually provided with a groove into which the stand fits so as to be held firmly in its place. Most commonly, however, nowadays as a precaution against accidents, the Host is not merely fixed between two crescent-shaped strips of metal but is enclosed in a pyx with two glass faces and this pyx is itself inserted bodily into the receptacle of the monstrance.