Generally, measuring instruments are meant to get the (pre-)existent value of a physical quantity of a system with as little change of, or interaction with the system as possible. — If a measuring instrument fixes a physical quantity of a physical system for further use (post-existent), then the measurement process is called preparation.
For example, a graduated cylinder filled to a definite mark prepares a certain volume of the liquid for further use.
Though preparation measures a physical quantity — at least by fixing it — it often destroys previous states of the physical system. This is usually experienced in quantum mechanics.