Variable

a measurable quantity which at every instant has a definite numerical value. If there is any doubt whether a particular quantity may be admitted as a variable, use the criterion whether it can be represented by a pointer on a dial. Pressure, angle, electric potential, volume, velocity, mass, viscosity, population, national income per capita and time itself, to mention only a few, can all be specified numerically and recorded on dials. Eddington's statement on the subject is explicit: "The whole subject matter of exact science consists of pointer readings and similar indications. Whatever quantity we say we are 'observing', the actual procedure nearly always ends in reading the pointer of some kind of indicator on a graduated scale or its equivalent." (Ashby, 1960, p. l5)