What is a substance that cannot be broken down into simpler substances?

Answer 1

A substance that cannot be broken down into simpler substances is a pure substance in its lowest form: an element. If if is an ionic compound, the lowest form would be an element. If it is a molecular compound or covalent compound, the lowest form would be a molecule.

Answer 2

Perhaps you are referring to an element? Not sure if that's what you're looking for, but in intro chemistry, that is what is usually taught.