Simple Defintion:
An enzyme is a type of protein that helps chemical reactions happen faster in living things [Protein is a type of molecule that is made up of building blocks called amino acids]. An enzyme is a thing in your body that helps your body break down food and do other things it needs to do. For example, enzymes in our stomach help break down food so we can digest it.
Very Simple Definition:
An enzyme is a thing in your body that helps your body break down food and do other things it needs to do. For example, when you eat an apple, enzymes in your body help break down the apple so your body can use it for energy.