A Colloid is basically a particle that has an electrical charge that can cause them to bind to each other.
Colloidal particles can be organic or inorganic. They can range in size from 0.01 - 1.0 micron. Colloids are quantified by measuring the rate at which water blocks a submicron filter. This can lower resistivity and incapacitate reverse osmosis membranes in deionized water systems.