An cannot be broken down into any other substance. element
compound
mixture
solution

Element

Explanation:

An Element cannot be broken down into any other substance or anything smaller. A Element is the Smallest Known Substance in the World.

According to Webster's Dictionary states "Each of more than one hundred substances that cannot be chemically interconverted or broken down into simpler substances and are primary constituents of matter. Each element is distinguished by its atomic number, i.e. the number of protons in the nuclei of its atoms."