A lock is a sluice constructed for boats, providing a gated channel of water allowing them to navigate the transition between two sections of water. In most cases these are used where the water level differs between the two sides, and while one or more boats is in the lock the gates will close and the water level inside is raised or lowered to match the water on the forward side.

The gates isolate the vessel within the water-filled chamber. Where the adjoining levels differ, changing the chamber's water level brings the vessel to the level needed for onward passage.

A ship elevator physically lifts or lowers the ship using a moving device. The lock instead enables the transition through its gated water chamber.