Berzelius (1836) named the property by which certain substances loosen bonds in reacting molecules and speed up a reaction, without themselves being permanently changed, catalysis (Greek: kata — wholly, lein — to loosen). The modern definition covers both directions of rate change: a catalyst alters (speeds up in positive catalysis, or slows down in negative catalysis) the rate of a reaction without undergoing any permanent chemical change itself.

Catalysed reactions split into two families by whether the catalyst shares a phase with the reactants. In homogeneous catalysis, reactants, products and catalyst are all in the SAME phase — e.g. $\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} + [\text{NO}]\text{(g)} \rightarrow \text{SO}_3\text{(g)} + [\text{NO}]\text{(g)}$, acetaldehyde's $\text{I}_2$-catalysed decomposition (all vapour phase), and the acid-catalysed hydrolyses of cane sugar and of ethyl acetate (all aqueous phase). In heterogeneous catalysis — also called contact catalysis — the catalyst is in a DIFFERENT phase from the reactants, typically a solid (often finely divided metal or gauze) acting on gaseous or liquid reactants: the Contact process ($\text{SO}_2 + \text{O}_2 \rightarrow \text{SO}_3$ over Pt or $\text{V}_2\text{O}_5$), the Haber process ($\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3$ over Fe), ammonia oxidation over Pt gauze, ethylene hydrogenation over finely divided Ni, $\text{H}_2\text{O}_2$ decomposition over Pt, and the AlCl3-catalysed acylation of benzene are all heterogeneous. …