A bulkhead is a wall or barrier inside a ship, submarine, or airplane that divides the interior into separate sections called compartments. Think of it like the walls in your house that separate different rooms, except bulkheads serve a critical safety purpose.

On ships, bulkheads are especially important because they can be sealed watertight. If the hull gets damaged and water starts flooding in, sailors can close heavy doors in the bulkheads to trap the water in just one compartment. This prevents the entire ship from flooding and sinking. The Titanic had bulkheads, but they weren't tall enough, so when the ship tilted, water spilled over the top from one compartment to the next. Modern ships have bulkheads that extend much higher.

Submarines rely on bulkheads to maintain air pressure in different sections and to survive underwater emergencies. In airplanes, a bulkhead separates the passenger cabin from the cockpit or cargo areas.

The word can also refer to a retaining wall that holds back earth or water, like the bulkheads built along shorelines to prevent erosion. You might see these wooden or concrete walls at beaches or riverbanks, standing firm against waves and currents.