A system of small solid particles dispersed in a liquid. Suspensions are a type of colloidal dispersion. By classical definition, suspension particles have diameters (or, if not spherical, the largest dimension) between 1 nanometer and 1 micrometer. However, for practical purposes the principles of colloid science can be usefully applied to suspensions for which the particles are smaller, down to about 0.1 nm (nanoparticle suspensions), and also to those for which the largest particle dimensions are tens or even hundreds of micrometers.

The particles in a suspension are large enough that they do not behave like the atoms and molecules of classical chemistry. For example, suspensions do not generally behave like true solutions, and may have undetectable freezing-point depressions. On the other hand, the particles are small enough that they do not behave like the macroscopic particles of classical physics. For example, sedimentation of a suspension's particles may occur extremely slowly in apparent violation of Stokes' law.