Composition of a substance
Composition of a mixture
The chemical composition of a mixture can be defined as the distribution of the single substances that constitute the mixture, called "components". In other words, it is defined giving the concentration of each component.
Because there are different ways to define the concentration of a component, as a consequence there are also different ways to define the composition of a mixture. For example it can be expressed as molar fraction, volume fraction, mass fraction, molality, molarity or normality.