What is the definition of chemical breakdown?

Answer 1

Generally it means breaking a larger molecule into smaller parts. That can be done by means of injecting energy into the molecule such that it breaks apart, or by introducing other molecules, ions, or atoms into the structure.

Energy could be thermal, you heat the chemical until it decomposes. It could be light such as UV, which is high energy and breaks chemical bonds between atoms, thus changing or breaking apart the molecule. It could be any part of the electromagnetic spectrum from gamma rays to radio waves, they are all energy.

The chemical could be exposed to ions from an acid or base - the ion attacks some of the chemical bonds in the molecule and breaks them. It could be exposed to a reactive molecule such as oxygen, which oxidizes the chemical (think iron to rust), thus causing breakdown.

Answer 2

This usually means a decomposition reaction, which is one of the 5 main reaction types. Typically in a decomp reaction you have more products than reactants, often one reactant and two or more products. An example is the decomposition of carbonic acid into water and carbon dioxide: H2CO3 --> H2O + CO2.