The term that refers to substances that cannot be broken down physically or chemically is element.

Here's why:

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

* Compounds are made up of two or more elements chemically combined in a fixed ratio. These can be broken down into their constituent elements.

* Mixtures are combinations of substances that are not chemically bonded. These can be physically separated.

Let me know if you have any other questions!