Chemically speaking, a catalyst is a substance that initiates or accelerates a reaction without itself being affected.
Which is correct, as far as it goes, but it's also a reductionist view.
The catalyst may appear unchanged from its initial state, but nevertheless it participates in the reaction. The reactants adhere to its surface, and squirm inside its pores. They shed an electron here, two there, dropping the detritus of their old form and using the catalyst to re-shape themselves into a new incarnation. They borrow the catalyst's stability and strength, plundering its interior as a means to an end.
Then they jump. Tearing themselves from the catalyst, dropping into the freedom of their shiny new form, they are butterflies climbing free of their caterpillar cocoons. They leave nothing behind, physically at least.
But the catalyst remembers. She lost some of herself, however briefly, to fund that transformation. They squirmed across the surface of her and burrowed into her capillaries, those caterpillars turned butterflies, those atoms in search of a new molecular pairing. It was by her power they were granted their new direction, and the strength and energy to pursue it.
They changed her, too, however briefly.