Metousiosis is a Greek term (μετουσίωσις) that means a change of οὐσία (essence, inner reality). Cyril Lucaris (or Lucar), the Patriarch of Alexandria and later of Constantinople, used this Greek term to express the idea for which the Latin term is transsubstantiatio (transubstantiation), which likewise literally means a change of substantia (substance, inner reality), using, in the 1629 Latin text of his The Eastern Confession of the Orthodox Faith, the term transsubstantiatio, and, in the Greek translation published in 1633, the term μετουσίωσις.

To counter the teaching of Lucaris, who denied transsubstantiatio/μετουσίωσις, Metropolitan Petro Mohyla of Kiev (also called Peter Mogila) drew up in Latin an Orthodox Confession, defending transubstantiation. Translated into Greek, using "μετουσίωσις" for the Latin term "transubstantiation", this Confession was approved by all the Greek-speaking Patriarchs (those of Constantinople, Alexandria, Antioch, and Jerusalem) in 1643, and again by the 1672 Synod of Jerusalem (also referred to as the Council of Bethlehem).