Fourth Lateran Council
Fourth Lateran Council (1215), Ecumenical council that first dogmatically used the term "transubstantiation". Wikipedia
"His body and blood are truly contained in the sacrament of the altar under the forms of bread and wine, the bread and wine having been changed in substance, by God’s power, into his body and blood. "
Why this quote matters
Doctrinal milestone introducing the term "transubstantiation."
Source citation: Decrees of the Ecumenical Councils, ed. N. Tanner (1990).