Photocatalysis is defined as "the acceleration of photoreaction speed due to the presence of a catalyst." A catalyst neither changes nor becomes consumed by a chemical reaction.

This definition includes photosensitization, a process in which a molecular species undergoes photochemical alteration as a result of an initial absorption of light energy by another molecular species, known as a Photocatalyst.

The chlorophyll of plants is a type of photocatalysis. In relation to photosynthesis, in which chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, photocatalysis (in the presence of a photocatalyst, light and water) generates a strong oxidizing agent capable of transforming organic substances into carbon dioxide and water.