Apure substance that cannot be broken down into any other substances by chemicals or physical mean is a

It is a element.

Pure substances includes either compunds, or elements. Elements are the simplest form of pure substance, they only consist of one kind of atoms. On the other hand, compounds contains two or more elements that are chemically combined together. Although you couldn't separate a compound by physical methods like filtration, but you can seperate them with chemical methods such as heating or electrolysis, using electricity.

Therefore, since compounds can be broken down eventually, so, the answer will be element.