Bentonite refers to a mixture of swelling clay minerals.  For a more technical definition ..

... containing at least eighty-five percent of the mineral montmorillonite (predominantly sodium montmorillonite) which meets the specifications of the most recent revision of API Standard 13A. 

Bentonite grout is a mixture of bentonite (API Standard 13A) and water in a ratio of not less than one pound of bentonite per gallon of water.