Instrumentation is the branch of science that deals with measurement and control in order to increase efficiency and safety in the workplace.
An instrument is a device placed in the field, or in the control room, to measure or manipulate flow, temperature, pressure and other variables in a process. Instruments include but are not limited to valves, transmitters, transducers, flame detectors and analyzers. Instruments send either pneumatic or electronic signals to controllers which manipulate final control elements (a valve) in order to get the process to a set point, usually decided by an operator.
Control instrumentation includes devices such as solenoids, Electrically Operated Valves, breakers, relays, etc. These devices are able to change a field parameter, and provide remote control capabilities.