A Catalyst is a substance that enables a speed up in a chemical reaction but is not consumed in the reaction; hence a Catalyst can be recovered chemically unchanged at the end of the reaction/ it is used to speed up or catalyse a chemical reaction. This means the mass of the Catalyst remains the same before, during and after the chemical reaction is completed. Thus, without the presence of the Catalyst the chemical reaction will be so slow it is unobservable, or the reaction will not speed up.
Technically as the Catalyst is not consumed in the process it is not involved in the actual chemical reaction but without its presence the increase in the reaction will not take place hence it is active in the chemical reaction. Yes, that is the oxymoron. But here is the answer to the oxymoron conundrum of not being involved in the chemical reaction but at the same time active in the chemical reaction. The Catalyst works by providing an alternative pathway for the reaction to take place. This alternative pathway has a lower activation energy than the pathway with the Catalyst. Because of the lower activation energy there are more molecules with the required activation energy thus more reactive collisions take place per second resulting in a faster reaction.
Catalysts are widely used in industrial processes to make such things as fertilisers and detergents. You may already be familiar with the word Catalyst because of Catalytic Converters uses in transport vehicles to remove toxin exhaust gases by converting them into harmless products.