A measure for something; a means of deriving a quantitative measurement or approximation for otherwise qualitative phenomena (especially used in engineering).
A function for the measurement of the "distance" between two points in some metric space: it is a real-valued function d(x,y) between points x and y satisfying the following properties: (1) "non-negativity": d(x,y) \ge 0 , (2) "identity of indiscernibles": d(x,y) = 0 \mbox{ iff } x=y , (2) "symmetry": d(x,y) = d(y,x) , and (3) "triangle inequality": d(x,y) \le d(x,z) + d(z,y) .
The modern version of that system, Systeme Internationale d'Unites (International System of Units), or SI system of measurements that is based on the base units of the meter/metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.