Transubstantiation (Latin: transubstantiatio; Greek: μετουσίωσις metousiosis) is, according to the teaching of the Catholic Church, "the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of the Blood of Christ". This change is brought about in the eucharistic prayer through the efficacy of the word of Christ and by the action of the Holy Spirit. However, "the outward characteristics of bread and wine, that is the 'eucharistic species', remain unaltered". In this teaching, the notions of "substance" and "transubstantiation" are not linked with any particular theory of metaphysics.

The Roman Catholic Church teaches that, in the Eucharistic offering, bread and wine are changed into the body and blood of Christ. The affirmation of this doctrine was expressed, using the word "transubstantiate", by the Fourth Council of the Lateran in 1215. It was later challenged by various 14th-century reformers, John Wycliffe in particular.

The manner in which the change occurs, the Roman Catholic Church teaches, is a mystery: "The signs of bread and wine become, in a way surpassing understanding, the Body and Blood of Christ." In Anglicanism, the precise terminology to be used to refer to the nature of the Eucharist has a contentious interpretation: "bread and cup" or "Body and Blood"; "set before" or "offer"; "objective change" or "new significance".

In the Greek Orthodox Church, the doctrine has been discussed under the term of metousiosis, coined as a direct loan-translation of transubstantiatio in the 17th century. In Eastern Orthodoxy in general, the Sacred Mystery (Sacrament) of the Eucharist is more commonly discussed using alternative terms such as "trans-elementation" (μεταστοιχείωσις, metastoicheiosis), "re-ordination" (μεταρρύθμισις, metarrhythmisis), or simply "change" (μεταβολή, metabole).