In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place.

An object which sinks completely displaces an amount equal to its volume; Archimedes' Principle states that when this happens, the weight of the object is reduced by its volume times the density of the fluid. If the weight of the object is less than this quantity, it will float; the amount of fluid displaced is directly related (via Archimedes' Principle) to its weight.

This is widely used in describing ships. The displacement of a vessel is defined as the weight of the amount of water it displaces when afloat. A ship's size is reported as the number of long tons of water it displaces; see dead weight tonnage. A variation of this meaning is a measure of shipping capacity.

Also, a displacement hull pushes its way through water, being supported entirely by displacement, as opposed to a hydroplaning hull, which glides over the surface of the water while being partially supported by hydrodynamic forces caused by the motion of the hull across the water, or a hydrofoil, which lifts the hull from the water entirely, using underwater wings.