A pyx is a small, round, sacred container used in the Catholic Church to carry the Eucharist to those who are unable to attend Mass. A pyx is used to bring the Eucharist to the sick or homebound so they can receive Holy Communion. Pyxes are often made of gold or silver-gilt, but can also be made of other precious materials like ebony. 
The word "pyx" comes from the Greek word pyxis, which means "boxwood receptacle". In the Middle Ages, the term was used more generally to refer to any vessel that held the Eucharist, including the cup that held the Blessed Sacrament above the altar.