Ships are measured in terms of overall length, length along the waterline, beam (breadth) and tonnage. There are a number of different tonnage definitions, the majority of which are measures of volume rather than displacement. Displacement is most frequently applied to naval vessels and is equal to the actual weight of a ship under specific conditions. "Light ship" tonnage is the actual weight of the ship with no fuel, no persons, no cargo, no water on board, just as it first entered the water. The term "displacement" is used because of the basic physical law, discovered by Archimedes, that the weight of a floating object is exactly that of the weight of the water that would otherwise be in the "hole in the water" created by the ship.