Definition and meaning of 'pyrolysis' can be understood by breaking the word into two parts: Pyro = heat & Lysis = break down.
Pyrolysis is breakdown of larger molecules into smaller molecules using heat or high temperature.
At any given temperature molecules are in vibrating stage. This is called molecular vibration. These molecular vibrations are directly proportional to the temperature of these molecules. During pyrolysis reaction, the molecules are subjected to very high temperatures leading to very high molecular vibrations. At these high molecular vibrations, every molecule in the object is stretched and shaken to such an extent that molecules start breaking down into smaller molecules.
The simplest example of pyrolysis is food cooking. When you cook food, the temperature of food increases. Higher temperatures lead to higher molecular vibrations which eventually breakdown larger complex food molecules into smaller and simple easy-to-digest molecules.
Plastic Pyrolysis involves the molecular breakdown of larger plastic molecules into smaller molecules of oil gas and carbon using heat. Pyrolysis is also known as thermal cracking, cracking, thermolysis, depolymerisation, etc.
When catalyst is used in the pyrolysis process, it is called catalytic pyrolysis.