jomifil, I did say that the definitions of substance and accident were philosophical definitons - they come from Aristotle, but I guess you can pick up the phone and tell him he was wrong. And I was reasonably sure I'd explained what the words meant in the context of transubstantiation.
Even in philosophical terms those words have changed their meanings over the years, however when the Catholic church finally settled on a definition of transubstantiatioin in the 1500's, the idea of it had been around, and the word itself had been in use, for several hundred years. As strange as it may seem, that was a period when philosophy, logic and science were all heavily influenced by the works of some ancient Greek guy called .. umm ... oh yes, Aristotle.
But since you like it plain and simple, the doctrine of transubstantiation comes out of a period when the meaning of substance was that used by Aristotle, not your 1951 dictionary. So for that doctine, it's Aristotle's meaning of the word which counts, along with the related meaning of accidents, not the one we use today.
But if you want modern substitutes for those terms, how about essence and attributes ? Perhaps not, essence can get slippery too I imagine.
In the end it all comes down to faith. For a Catholic who accepts the teachings of the church, then for them, at the moment of consecration, the bread and wine become the body and blood of Christ. They may look, feel, smell and taste like bread and wine, but for the Catholic their fundamental nature has been changed.
Someone who isn't a Catholic may well think that the Catholics are being conned, flim-flamed, fooled, taken for a ride or whatever other way you might choose to put it, but as the beliefs of Catholics don't have any significant effect on the lives of those who aren't, does it really matter ?