Simple Definition:
A molecule is the smallest unit of a substance that retains all of the chemical properties of that substance. It is made up of two or more atoms that are chemically bonded together. For example, a molecule of water is made up of two hydrogen atoms and one oxygen atom (H2O).
Very Simple Definition:
A molecule is a tiny particle that makes up everything around us, including the air we breathe and the water we drink. For example, a water molecule is made up of two hydrogen atoms and one oxygen atom.