set of metabolic pathways that breaks down molecules into smaller units

Catabolism is a kind of metabolism

Catabolism is what happens when you digest food and the molecules break down in the body for use as energy. Large, complex molecules in the body are broken down into smaller, simple ones. An example of catabolism is glycolysis. This process is almost the reverse of gluconeogenesis.

And it's the process of breaking the chemical bonds among the atoms of macromolecules to extract the chemical energy that is stored in them, such as releasing the energy that results from the glucose oxidation (during Cellular respiration process).