A submarine is a watercraft capable of independentoperation below the surface of the water. It differs from asubmersible, which has only limited underwater capability. The term submarine most commonly refers to large crewedautonomous vessels; however, historically or more casually, submarine can also refer to medium sized or smaller vessels(midget submarines, wet subs), Remotely Operated Vehiclesor robots. The word submarine was originally an adjectivemeaning "under the sea", and so consequently other usessuch as "submarine engineering" or "submarine cable" maynot actually refer to submarines at all. Submarine wasshortened from the term "submarine boat", and is oftenfurther shortened to "sub".