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 word submarine was originally an adjective meaning "under the sea"; consequently other uses such as "submarine engineering" or "submarine cable" do not necessarily refer to the vessel. Submarine was in fact shortened from the proper term, "submarine boat", and is often further shortened to "sub" when the word is employed informally. Submarines should always be 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, and they were adopted by several different navies. Submarines were first widely used during World War I (1914–1918) and now feature in many large navies. Military usage includes attacking enemy surface ships or submarines, aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional land attack (for example using a cruise missile), and covert insertion of special forces. Civilian uses for submarines include marine science, salvage, exploration and facility inspection/maintenance. Submarines can also be modified to perform more specialized functions such as search-and-rescue missions or undersea cable repair. Submarines are also used in tourism, and for undersea archaeology.