A mixture of tiny particles of one substance, called the dispersed phase, suspended in another substance, called the dispersion medium. The particles are so small that they remain in suspension indefinitely, unaffected by gravity. Both the dispersed phase and the dispersion medium may be solid, liquid, or gaseous, although the dispersal of one gas in another is not known as colloidal dispersion.

An aerosol is a colloidal dispersion of either a solid or a liquid in a gas. An emulsion is a colloidal dispersion of a liquid in another liquid. A sol is a colloidal dispersion of solid particles in a liquid. A gel is a sol in which the suspended particles are organized in a loose but definite three-dimensional arrangement, giving some rigidity and elasticity to the mixture.

Because of their small size, colloidal particles can pass through ordinary filters, but not through the extremely fine openings in a semipermeablemembrane, such as parchment. Although individual colloidal particles are too small to be seen with an ordinarymicroscope, they can be made visible by means of an ultramicroscope, or dark-field microscope. The particles are directly visible in an electron microscope.