A torpedo is a self-propelled naval missile (guided or unguided) that travels underwater to it's target. The term originally applied to static devices that we would call "sea mines" today. Giovanni Luppis first demonstrated an unmanned, self-propelled explosive boat in 1860, and he worked with Robert Whitehead to develop an actual weapon that Whitehead successfully demonstrated in 1866. The term became associated with the new weapon. The principal value of a torpedo is that it can easily damage a sea vessel below the water line.
Torpedoes had several impacts on naval warfare. In the late 19th century, a new type of warship emerged, the Torpedo Boat, starting with HMS Lightning in 1876. These small, cheap, low-range, and quick ships were armed with a small number of torpedoes, which made them capable of inflicting significant damage on heavily armed and armored battleships and cruisers. Since they posed such a threat to these expensive ships and could be made cheaply by smaller-scale and less-sophisticated shipyards, a new class of ship was created to deal with them: the Torpedo Boat Destroyer. The advent of reliable torpedoes also made smaller caliber guns on battleships (which were quick-firing but had limited firepower and range, as well as requiring their own specialized ammunition) obsolete. Torpedoes would later be used as the main weapons of submarines and, later still, torpedo bombers. The latter lead to the end of the era of the battleship.
In modern navies, torpedoes can be launched from ships or submarines or dropped from aircraft. Torpedoes can also be equipped with rockets so they can fly through the air to the vicinity of the target before entering the water and using SONAR to home in on the target.
The word torpedo originally descends from a variety of electric ray (fish).
Torpedoes in Science Fiction
In Science Fiction, the term torpedo is often used to describe space-based, anti-starship missiles.