Which term describes the breaking down of body cells or substances?

The term which describes the breaking down of body cells or substance is CATABOLISM.
The metabolic processes which occur in living things are of two types, these are anabolic and catabolic processes. The anabolic process involves the building up of needed materials in the body while the catabolic process involves the breaking down of waste materials in the body in order to eliminate them through excretion.

catabolism This is a one part of metabolism, the other part is known as anabolism. Catabolism breaks down larger units into small ones , these can be amino acids, polysaccharides and even fatty units in the body. The products of the breakdown is either energy or another substantial end product for another process useful in the body