A substance that cannot be broken down into simpler substances by chemical or physical means is called

A substance that cannot be broken down into simpler substances by chemical or physical means is called an element.

Explanation:

An element is the 'purest and simplest' form of matter or a substance.

An element is considered pure because all of its atoms are of a similar kind with a unique set and same defined number of protons. The atoms of an element exhibit the same set of properties. They can neither be broken down further into simple components nor can be converted to another element. Similarly, elements also cannot be destroyed or created by a chemical reaction. There are around 118 elements like oxygen, hydrogen, carbon, gold, iron, copper, platinum etc.