Submarine

1. Generally, a watercraft capable of independent operations underwater, able to renew its own power and breathing air. A submarine differs from a submersible, which has more limited underwater capabilities. By naval tradition, any submarine is referred to informally as a "boat" regardless of its size.
2. Most commonly, a large, crewed vessel capable of independent underwater operations.
3. Historically and colloquially, a broad category of vessels capable of submerged operations, including large, crewed submarines but also medium-sized and smaller vessels such as midget submarines and wet subs and vessels technically considered submersibles because they require external support, such as remotely operated vehicles and autonomous underwater vehicles.