a) Mineralogical/Petrological term: A soft, plastic, light-colored rock composed primarily of clay minerals of the smectite group, particularly the clay mineral montmorillonite, which typically forms from chemical alteration of glassy volcanic ash or tuff under marine or hydrothermal conditions. Bentonite may contain accessory crystal grains that were originally phenocrysts in the parent rock as well as secondary authigenic mineral phases such as K- rich feldspar. Diagenetic or low-grade metamorphic alteration can modify the smectite to a variety of interstratified illite-smectite minerals, resulting in materials known as K-bentonites.

b) Industrial term: A highly colloidal and plastic clay material, primarily composed of the clay mineral montmorillonite, that is commonly used in drilling mud, as a foundry sand binder, in cat litter, animal feed, cements, ceramics and various other industrial activities and products. Sodium bentonite swells significantly when exposed to water (to ~12X) whereas calcium bentonite has minimal swelling capability (to ~3X).
Cf., tonstein