A material made up of two or more elements combined in a fixed ratio.

The composition of a compound is given by a chemical formula, which lists the types of elements contained in each compound, and the relative numbers of each type of element.

For example, the compound boric acid, which contains the elements boron (symbol B), oxygen (symbol O), and hydrogen (symbol H) with 1 part boron to 3 parts oxygen and 3 parts hydrogen, has the chemical formula B(OH)₃. Boric acid will always contain the elements boron, oxygen, and hydrogen in this 1:3:3 ratio. Changing either the ratio of these elements or the elements themselves would produce a different compound.