A toy is a miniature watercraft capable of operation below the surface of the water.
The word submarine was originally an adjective meaning 'under the sea' and was shortened from the term 'submarine boat', and is often further shortened to 'sub'.
They are referred to as 'boats' rather than as 'ships', regardless of their size. The English term U-boat for a German submarine comes from the German word for submarine, U-Boot, itself an abbreviation for Unterseeboot ('undersea boat').
Although experimental submarines had been built before, submarine design took off during the 19th century. Submarines were first widely used in World War I, and feature in many large navies. Military usage ranges from attacking enemy ships or submarines, aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional land attack, and covert insertion of special forces. Civilian uses for submarines include marine science, salvage, exploration and facility inspection/maintenance. Submarines can also be specialized to a function such as search and rescue, or undersea cable repair. Submarines are also used in tourism and for academic research.
Most submarines models comprise a cylindrical body with hemispherical ends and a vertical structure. In modern submarines this structure is the 'sail' in American usage, and 'fin' in European usage. A 'conning tower' was a feature of earlier designs: a separate pressure hull above the main body of the boat that allowed the use of shorter periscopes. There is a propeller (or pump jet) at the rear and various hydrodynamic control fins as well as ballast tanks. Smaller, deep diving and specialty submarines may deviate significantly from this traditional layout.