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Submarine (or submarine for short) is a kind of vessel that can operate independently under water. It is different from a submersible, which has limited underwater capability. The term most commonly refers to large manned ships. It has also been used historically or colloquially to refer to remote-controlled vehicles and robots, as well as medium-sized or smaller vessels, such as small submarines and submarines. The term submarine is a simplified form of submarine. According to naval tradition, submarines are often referred to as "boats" rather than "boats", regardless of their size (boats are usually reserved for smaller ocean going vessels).

Although experimental submarines were built before, the design of submarines began to rise in the 19th century and was adopted by some navies. Submarines were widely used during World War I (1914 – 1918) and are now used in many sizes of the Navy. Military uses include attacks on hostile surface ships (merchant and military), attacks on other submarines, carrier protection, blockade operations, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional ground attacks (such as the use of cruise missiles), and covert insertion into special forces. The civilian uses of submarines include marine science, salvage, exploration, and facility inspection and maintenance. Submarines can also be retrofitted to perform more specialized functions, such as search and rescue missions or submarine cable repairs. Submarines are also used in tourism and undersea archaeology.

Most large submarines consist of a cylinder with a hemispherical (or conical) end and a vertical structure usually located in the middle of the hull, which is equipped with communication and sensing devices and periscopes. In modern submarines, this structure is "sail" when used in the United States and "fin" when used in Europe. The "cone tower" is an early design feature: a single pressure hull above the main body of the ship allows the use of shorter periscopes. There is a propeller (or pump nozzle) at the rear and various hydrodynamic control fins. Small, deep submergence and special submarines may be quite different from this traditional layout. Submarines use submarines and also change the amount of water and air in the ballast tanks to change buoyancy to submerge and surface.

Submarine is one of the most widely used types and capabilities of any ship. They range from small autonomous cases to one or two man ships that can run for hours, to ships that can submerge for six months, such as the Russian typhoon, the largest submarine ever. Submarines can survive or be used in more depth than human divers. Deep submersibles are derived from deep submarines, and deep submarines are evolved from diving bell.