The word photocatalysis is composed of two parts:
The prefix photo, defined as “light”,
Catalysis is the process where a substance participates in modifying the rate of a chemical transformation of the reactants without being altered in the end. This substance is known as the catalyst which increases the rate of a reaction by reducing the activation energy.
Hence, photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself.