Answer:
The term "instrumentation measurement system" refers to a collective set of measuring instruments used for indicating, measuring, and recording physical quantities. An instrumentation system exists to provide information about the physical value of some variable being measured. These variables, also known as process variables, state a condition of the process fluid (a liquid or gas) that can change the manufacturing process in some way. The objective of any measurement endeavor is to be able to measure a given process variable in order to possibly control it. Hence, what you cannot measure, you cannot control. Instruments are used to measure and control the condition of process streams as they pass through a plant.
Source: Wikipedia, The Instrument Guru, Instrumentation Toolbox
