the.com/transubstantiation

means The doctrine, central to Catholic belief, that during the Eucharist the bread and wine are wholly converted into the body and blood of Christ, even though every physical property — taste, texture, appearance — stays exactly the same.

from Built from Latin trans- ('across, change') plus substantia ('substance, that which stands under'), itself from sub- ('under') and stare ('to stand'). The term crystallized in medieval theology, drawing on Aristotle's distinction between 'substance' (a thing's true underlying nature) and 'accidents' (its outward, sensible qualities) — letting scholars say the substance crosses over to become Christ while the accidents of bread and wine remain untouched. The Fourth Lateran Council, in the early 1200s, affirmed the term, and it was later sharpened in debate during the Reformation, when rival views of the Eucharist made the precise word suddenly matter a great deal.