A pyx or pix (Latin: pyxis, transliteration of Greek: pyxis, box-wood receptacle, from pyxos, box-tree) is a small container used in the Roman Catholic, Old Catholic and Anglican Churches to carry the consecrated Host (i.e., the consecrated Eucharist), to the sick or invalid or those otherwise unable to come to a church in order to receive Holy Communion.
The word pyx comes from the Greek word πυξίς, "pyxis" meaning box or receptacle. The plural is pyxides. While the word may be applied to any covered carrier, in the modern usage the term is usually applied to small, flat, clamshell-style containers often about the size of a pocket watch and usually made of brass or other metals, traditionally lined with gold. A fabric or leather pouch in which the pyx may be carried is known as a burse. Typically, this kind of burse can be securely closed and is fixed with cords so that the priest or other Eucharistic Minister can affit it to his or her person during transport to prevent the consecretated host(s) from being accidentally lost.
The term pyx is also a standard term used in the Roman Catholic Church to refer to a flat, circular container, sometimes called a lunette, composed of a ring of metal (usually lined with gold) holding two glass or crystal disks, to create a round, flat, glass-enclosed space for the Eucharistic Host. This is used together with a monstrance for exposition and Benediction services. The lunette is often kept in another object, itself sometimes called a pyx, luna, or custodia, which is usually a round box often on a small stand, giving the impression of a faceless, old-fashioned, alarm clock.
All of these objects, whenever they contain a consecrated host, are normally kept in a safe or cabinet within the tabernacle. The tabernacle may reside either behind the main altar or within a special Eucharistic chapel.
In late antiquity, the custom developed in the East of suspending a vessel in the form of a dove (Greek: peristerion, Latin: peristerium) over the altar and used as a repository for the Blessed Sacrament. This custom is mentioned by Gregory of Tours in his Life of St. Basil, and in several ancient French documents. The custom probably came to France from the East, for it never seems to have existed in Italy. Examples of this practice may still be found in use today; for instance, in the Cathedral of the Dormition in Moscow.