This is a Christian religious term, and a very specific one. In church, at the service called Holy Communion (or Mass), the priest gives out bread and wine. Transubstantiation is the teaching — held by the Roman Catholic Church, and by the Orthodox churches in their own way — that the bread and wine truly become the body and blood of Christ. Not a picture of it, not a reminder of it: really become it, even though to your eyes, your tongue and your nose nothing at all has changed. That last part is the whole idea. The old thinking behind it says a thing has an inner reality ('substance') and an outer appearance; here the inner reality changes while the appearance stays. This is one of the big dividing lines between Catholic and Protestant Christianity — most Protestant churches say the bread and wine stand for the body and blood, and they reject this word. So it is a formal, technical word: you meet it in religion class, in history (arguments about it got people killed in the 1500s), and in church writing. It has no everyday use. Very occasionally a writer will borrow it as a joke or an image for a total change of one thing into another, but that is a stretch, and readers will hear the church meaning first.

變體論／聖餐變體（天主教教義：聖餐的餅酒實質變成基督的身體與血）