Peral was the first successful full electric battery-powered submarine, built by the Spanish engineer and sailor Isaac Peral for the Spanish Navy, in Arsenal de la Carraca. The first fully capable military submarine, she was launched 8 September 1888. She had one torpedo tube and an air regeneration system. Her hull shape, propeller, periscope, torpedo launcher and cruciform external controls anticipated later designs. Her underwater speed was 3 knots. With fully charged batteries, she was the fastest submarine yet built, with underwater performance levels that matched those of First World War U-boats for a very short period, before her batteries began to drain. For example, the SM U-9, a pre-war German U-boat built in 1908, had an underwater speed of 8.1 knots, and an underwater range of 150 km (81 nmi) at 5.8 knots, before having to resurface to recharge her batteries. Although advanced in many ways, Peral lacked a means of charging batteries while underway, such as an internal combustion engine, thus had a very limited endurance and range. In June 1890, Peral's submarine launched a torpedo while submerged. It was also the first submarine to incorporate a fully reliable underwater navigation system. However, conservatives in the Spanish naval hierarchy terminated the project despite two years of successful tests. Her operational abilities have led some to call her the first U-boat.

A submarine is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely operated vehicles and robots, as well as medium-sized or smaller vessels, such as the midget submarine and the wet sub. Submarines are referred to as boats rather than ships irrespective of their size.