Element
Definition
A pure substance made of only one kind of atom. Oxygen, iron, and gold are elements: no chemical reaction can break them into simpler substances.
Technical definition
A chemical element is a pure substance whose atoms all share the same atomic number, meaning the same proton count. It cannot be decomposed into simpler substances by ordinary chemical reactions. 118 elements are currently recognized.
Example
Copper wire is made of the element copper. No chemical reaction can turn copper atoms into a different element.
Common confusion
Elements are not the same as compounds or mixtures. Water looks pure but is a compound of hydrogen and oxygen, not an element.