The term 'UNDERSEA WEAPON' can be summarized by the crossword solution of 'TORPEDO', which is a self-propelled, guided missile that is launched underwater to attack ships or other targets. Torpedoes are typically propelled by an electric motor and guided by an internal navigation system. They are designed to detonate on contact with their target, or in proximity to it, depending on the type of warhead. Torpedoes have been used since the late 19th century and remain an important part of modern naval warfare.