Science of measurement .
Measuring a quantity means establishing its ratio to another fixed quantity of the same kind, known as the unit of that kind of quantity. A unit is an abstract idea, defined either by reference to a randomly chosen material standard or to a natural phenomenon. For example, the metre , the standard of length in the metric system , was formerly defined (1889–1960) by the separation of two lines on a particular metal bar, but it is now defined as the distance traveled by light in a vacuum in 1/299,792,458 second. See also International System of Units .