Monstrance. From the Latin word ostendère (=show), an Ostensorium (or Ostensory) or a Monstrance  is the vessel used for the solemn exposition (ostension) of the Body of Christ (the consecrated Host) for Eucharistic adoration and blessing in Roman Catholic Church. It is usually forged in gold, silver and/or other precious metals and is formed by a pedestal supporting a sunburst with a glass case in the centre, where the lunette with the consecrated Host is placed. 

So that it is visible to the faithful for Eucharistic adoration, the Monstrance is placed on the altar.