An anti-submarine weapon is any one of a range of devices that are intended to act against a submarine, and its crew, to destroy (sink) the vessel or to destroy or reduce its capability as a weapon of war. In its simplest sense an anti-submarine weapon is a projectile

Projectile

A projectile is any object projected into space by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more commonly refers to a weapon....

, missile

Missile

Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 or bomb

Bomb

A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

 that is optimized to destroy submarines

Anti-submarine warfare

Anti-submarine warfare is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and deter, damage or destroy enemy submarines....

.
Prior to about 1890, naval weapons were used only against surface shipping. With the rise of the military submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 after this time, countermeasures were considered for use against them.