/ˌkɒnsəbˌstænʃiˈeɪʃən/
From the Latin con- ("with") and substantia ("substance"). The term was developed in the Middle Ages to describe the Lutheran view of the Eucharist, contrasting with the Roman Catholic doctrine of Transubstantiation.
Consubstantiation is a theological doctrine that describes the nature of the Eucharist. It holds that in the sacrament, the substance of the body and blood of Christ are present "in, with, and under" the substance of the consecrated bread and wine. This view maintains that the bread and wine are not transformed into the body and blood of Christ (as in transubstantiation), but that the communicant receives both the physical elements and the true, substantial presence of Christ's body and blood together. This is often called the "Sacramental Union."
CONSUBSTANTIATION, n. The union of the body and blood of Christ with the sacramental elements, according to the Lutherans, who deny transubstantiation.
The modern corruption is often not a direct alteration of the doctrine but its complete neglect or reduction to mere symbolism. Many evangelical circles have abandoned any notion of a "real presence" of Christ in the Lord's Supper, viewing it as a purely memorial act with no special grace conveyed. This reduces the sacrament to a cognitive exercise, stripping it of the mystery, power, and assurance that the historic Church, including the Reformers, believed Christ intended to provide through His physical presence in the elements.