Matter that cannot be broken down into anything simpler by chemical reaction

element

Explanation:

Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: water

Thus as it given that it cannot be broken down into anything simpler by chemical reaction , it is an element.