A compartment is a portion of the space within a ship defined vertically between decks and horizontally between bulkheads. It is analogous to a room within a building, and may provide watertight subdivision of the ship's hull important in retaining buoyancy if the hull is damaged. Subdivision of a ship's hull into watertight compartments is called compartmentation.

Bulkhead watertight compartments were invented by the Chinese which strengthened the junks and slowed flooding in case of holing during the Han and Song dynasties.[1][2][3][4] The wide application of Chinese watertight compartments soon spread to the Europeans through the Indian and Arab merchants.[5][6]

The economics of early unsinkable passenger ships was scrutinized in an 1882 Scientific American article