Solution

A Catalyst is a substance which accelerates the rate of a chemical reaction without undergoing any change in its chemical composition or mass during the reaction. A catalyst accelerates the rate of a reaction by lowering the activation energy.
Catalysts are of 2 types
i) Homogeneous catalyst: When reactant and catalyst are in same phase. In this catalysis, the catalyst combines with one of the reactants to form some intermediate compound, which is unstable and decomposes easily to form the product.

ii) Heterogeneous catalyst: When reactant and catalyst are in different phases. These are surface catalyst which acts through adsorption.

So, homogeneous catalysis has the catalyst in the same phase as the reactants. Typically everything will be present as a gas or contained in a single liquid phase.