An onomatopoeiac verb describing the movement of a liquid in a container in response to movement of, or forces exerted on the container.
The most familiar examples to most of us will be automotive fuel tanks, drinks bottles and the like. Industrial strength examples include aircraft fuel tanks, oil tankers etc, where sloshing liquid can be a problem: in aviation, sloshing fuel in large tanks can cause problems with balance, potentially causing deviations in flight attitude.
In oil tankers, since such a significant proportion of the vessel's mass is made up of the liquid it's transporting, sloshing can cause serious problem not only with balance, but the uneven weight distributions caused could create excessive structural loads on the structure of the ship..
Sloshing is typically overcome with the use of internal bulkheads to divide liquid compartments into smaller compartments (liquid being free to move only within the smaller compartments), or sponge or wick type materials which keep the distribution of the liquid more or less even, and impede its movement.
The sloshing behaviour of liquids, while in itself a fairly simple CFD problem, is difficult to model completely because it is effected by, and effects, the behaviour of it's container: modelling the motion of the oil in a tanker completely also requires modelling the behaviour of the vessel's hull in the water in response to the uneven forces applied by the liquid.