The substance that cannot be broken down into other substances is called an element.

Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom.

* Atoms are the basic building blocks of matter. They cannot be broken down further by chemical means.

For example, gold (Au) is an element. It's made up only of gold atoms. You can't break down gold into anything simpler without changing it into a different substance.