Catholics use the word "transubstantiation" to describe the "real presence" of Christ in the eucharist. When did this begin?
--Timing in Trenton
This word, and the language to describe Christ's "real presence" in the eucharist, does not occur before 12th century in Christian teaching.
The best place to get the "official word" on transubstantiation is the 13th session [11 October 1551] of the Council of Trent. On that day, chapter 4 of the decree on "real presence" talks about transubstantiation and ends the chapter with the words "This change the holy Catholic Church properly and appropriately calls transubstantiation" [in Latin Quae conversio convenienter et proprie a sancta catholica ecclesia transsubstantiatio est appellata].