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One meaning of displacement is a vector quantity which expesses the length and direction of a straight line from one place to another (as opposed to the scalar quantity distance which expresses only the length). The SI unit for either distance or displacement is the meter.

Another group of meanings of displacement is related to the measurement of a volume. The displacement of a ship is its weight, which is directly related (via Archimedes' Principle) to the amount of water it displaces when it floats. The displacement of an Otto or Diesel engine is the total amount of air it can draw in during one complete engine cycle; in a piston engine, this can be calculated by multiplying the number of cylinders by the area of a piston by the length of the stroke.

A third meaning of displacement, derived from the second, is a measure of shipping capacity. A ship's size is reported in the number of long tons of water it displaces. See dead weight tonnage.