Substance that is dispersed rather than necessarily dissolved within another substance.
In biology a typical colloidal substance is a protein suspended within an aqueous solution where the protein, being quite large, technically is not dissolved in the water. Instead it is partially dissolved (especially the exterior) and partially not so dissolved (especially the interior). These differences along with the protein's overall mass create a situation where separation from the water via centrifugation is readily achieved.
The presence of proteins in water gives rise to what is known as the Tyndall effect which is a scattering of light, by these colloids, such that overall the fluid takes on a bluish coloration. If you have ever wondered why skim milk is slightly blue in color, well, now you know!
Note that there are numerous additional examples of colloids beyond proteins in aqueous solutions and indeed the suspending medium need not even be liquid but instead can be gaseous and even a solid.
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