By “transubstantiation” I mean St. Thomas Aquinas’s description (I include the Latin only for those who may want to nitpick what St. Thomas meant, or what words he actually used):
“Et ideo cuiuslibet agentis creati actio fertur super aliquem determinatum actum. Determinatio autem cuiuslibet rei in esse actuali est per eius formam. Unde nullum agens naturale vel creatum potest agere nisi ad immutationem formae. Et propter hoc omnis conversio quae fit secundum leges naturae, est formalis. Sed Deus est infinitus actus, ut in prima parte habitum est. Unde eius actio se extendit ad totam naturam entis. Non igitur solum potest perficere conversionem formalem, ut scilicet diversae formae sibi in eodem subiecto succedant, sed conversionem totius entis, ut scilicet tota substantia huius convertatur in totam substantiam illius. Et hoc agitur divina virtute in hoc sacramento. Nam tota substantia panis convertitur in totam substantiam corporis Christi, et tota substantia vini in totam substantiam sanguinis Christi. Unde haec conversio non est formalis, sed substantialis. Nec continetur inter species motus naturalis, sed proprio nomine potest dici transubstantiatio.”
"For it is evident that every agent acts according as it is in act. But every created agent is limited in its act, as being of a determinate genus and species: and consequently the action of every created agent bears upon some determinate act. Now the determination of every thing in actual existence comes from its form. Consequently, no natural or created agent can act except by changing the form in something; and on this account every change made according to nature’s laws is a formal change. But God is infinite act, as stated in I, 7, 1; 26, 2; hence His action extends to the whole nature of being. Therefore He can work not only formal conversion, so that diverse forms succeed each other in the same subject; but also the change of all being, so that, to wit, the whole substance of one thing be changed into the whole substance of another. And this is done by Divine power in this sacrament; for the whole substance of the bread is changed into the whole substance of Christ’s body, and the whole substance of the wine into the whole substance of Christ’s blood. Hence this is not a formal, but a substantial conversion; nor is it a kind of natural movement: but, with a name of its own, it can be called ‘transubstantiation.’ " (Summa Theologica, Part 3, Q. 75, Art. 4)
Is it necessarily true, and what arguments are there for it being true?
Is it dogma, or theology (a not-necessarily-exclusive way to explain the dogma)?
Finally, why should we use the word, if Christ did not (and doesn’t clearly explain what is happening during communion)?