Software measurement once an obscure and esoteric speciaity, has become essential to good software engineering.
Measurement is 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.An entity is an object (eg. person, room) or an event (eg. journey, testing phase) in the real world.
An attribute is a feature or property of an entity (eg. color of a room, cost of journey, elapsed time of testing).
We thus measure attributes of entities
The definition of measurement is not clear-cut: Is "blue" a measure of the color of a room? Height is an attribute of a person that can be measured, but what about intelligence? Accuracy of measurement? Errors in measurement? What scale to use, m or cm? What calculations on results of measurements (eg. averages) are valid?
Measurenment is direct quantification, calculation is indirect.
"What is not measurable make measurable" (Galileo Galilei 1564-1642). Measurement makes concepts more understandable leading to better measurements and so on.
Examples IQ: intelligence of a person. HCP: skill in playing golf of a person, decathlon points: overall score in the ten atletics events by a person.