A thick fluid like a jelly, formed by the addition of a substance to water: A hydrogel is a colloidal gel in which water is the dispersion medium.

Hydrogel is a network of polymer chains that are water-soluble, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are superabsorbent (they can contain over 99% water) natural or synthetic polymers.

Some uses for hydrogel include disposable diapers that "capture" urine, or in sanitary towels, and for use with contact lenses (silicone hydrogels, polyacrylamides).