A torpedo is a self-propelled underwater missile designed to explode when it hits a ship. Unlike a bomb dropped from above, a torpedo travels through the water under its own power, using a propeller and steering mechanism to hunt down its target. Most torpedoes strike below the waterline, where a ship's armor is weakest, often causing severe flooding.

Torpedoes revolutionized naval warfare when they appeared in the late 1800s. Before torpedoes, warships had to get close enough to blast each other with cannons. Torpedoes allowed smaller, faster vessels called torpedo boats (and later, submarines) to threaten even the mightiest battleships. During World War II, submarine-launched torpedoes sank thousands of ships, making them one of the war's most feared weapons.

In casual speech, to torpedo something means to destroy or ruin it completely: “His careless comment torpedoed any chance of winning the debate.” Just as a torpedo can sink a mighty ship, a bad decision or unfortunate mistake can torpedo your plans or someone's reputation.