Dictionary.com defines a dispersion medium as the continuous medium, such as a gas, liquid or solid, in which a disperse phase is distributed. It can also be referred to as an external phase.
The term also refers to a substance in which another is colloidally dispersed. This is also known as a dispersing phase.
Colloids, also known as colloidal particles, are small particles found in solutions and suspensions. They can be mixed in such a way that they remain indefinitely dispersed. The resulting mixture is a colloidal dispersion, of which milk of magnesia is an example. It consists of solid magnesium hydroxide as the dispersed phase and water as the dispersion medium.