What is bentonite?

Bentonite is described as a clayey mineral formed by weathering of volcanic ash.
It gets its name from the place where it was first discovered: Fort Benton, Montana (USA). Its ability to absorb large amounts of liquid is due to the clay mineral montmorillonite. The name montmorillonite derives from the town of Montmorillon in southern France, where this mineral can also be found.

However, it is not the case that anything that clumps together can be called bentonite. According to the generally held definition, bentonite must comprise at least 50% montmorillonite. (Therefore, clumping litter is not the same as bentonite litter!)