The practice of mixing a little water with the wine that will be consecrated at the eucharist. The BCP states that this practice "is customary" (p. 407). It probably is derived from ancient Jewish custom. It was likely observed by Jesus at the Last Supper. It was the invariable practice of the early church, and it is referred to in the Apostolic Constitutions. However, it was rejected by Luther at the Reformation and not practiced in the Calvinist tradition. It was directed by the 1549 BCP but not mentioned in the 1552 Prayer Book. It is widely practiced now. It has been described as a sign of union of Christ with his people, a sign of the flow of blood and water from Jesus' side at the crucifixion, and a sign of the union of Christ's divine and human natures. In the eastern tradition, hot water is added to the chalice after the breaking of the bread to symbolize the descent of the Spirit and the vibrant energy of faith. This practice is known as the zeon.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.