Question

what is a catalyst defines as

Answer 1

Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, which is not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only very small amounts of catalyst are required to alter the reaction rate in principle

Answer 2

Some Chemical Reactions needs a catalyst which accelerates and decelerates the speed of the Reaction, and thus known as catalyst.
For Example : 2KClO3 --------------> 2KCl + 3O2
(MnO2 as catalyst)
I hope that you have got your answer.