The process by which numbers or symbols are assigned to attributes of entities in the real world in such a way as to describe them according to clearly defined rules[Fenton&Pfleeger 1996]
The experimental process in which, to precisely describe the entities or events in real world, numbers or other symbols are assigned to its attributes by using a given scale. The result of the measurement is called a measure. [Abreu&al 2000]
The act of determining a measure. [IEEE Std 729-1993]