The Catholic doctrine that during the Eucharist, the bread and wine are transformed into the actual body and blood of Christ while retaining their physical appearance. The substance changes completely while the accidents (appearance) remain the same.
From Latin 'trans' (across) and 'substantia' (substance), coined in medieval scholastic theology. The doctrine was formally defined at the Fourth Lateran Council in 1215 CE, using Aristotelian philosophical categories to explain the Eucharistic mystery.
Medieval theologians borrowed Aristotle's distinction between 'substance' and 'accidents' to solve Christianity's most mysterious ritual—how bread could literally become flesh while still looking like bread! This philosophical solution sparked the Protestant Reformation 300 years later.
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