A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term submarine most commonly refers to a large crewed autonomous vessel. However, historically or colloquially, submarine can also refer to medium-sized or smaller vessels (midget submarines, wet subs), remotely operated vehicles or robots. The adjective submarine, in terms such as submarine cable, means 'under the sea'. The noun submarine evolved as a shortened form of submarine boat (and is often further shortened to sub). For reasons of naval tradition submarines are usually referred to as 'boats' rather than as 'ships', regardless of their size.
Contributions by Pedant, Nautical, and The Epopt.