Colloid, KOL oyd, is a material composed of tiny particles of one substance that are dispersed (distributed), but not dissolved, in another substance. The mixture of the two substances is a colloidal system. A colloidal system composed of solid or liquid particles dispersed in a gas is called an aerosol (see Aerosol). A system made up of solid or liquid particles in water is sometimes called a sol or a hydrosol. The word colloid is often used alone to mean colloidal system. The remainder of this article uses colloid in this way.