Instrumentation is defined as the art of science in measurement and control 1.
Instruments can be found anywhere, they can be found in laboratories, refineries, factories, vehicles and even at school and in our homes too. For example, the faucet which Sarah quoted as the saddest instrument controls the flow of water flowing to the kitchen sink or a glass. In western homes smoke detector is also a common instrument.
In the plant industries, Instrumentation is the art of measuring the value of some plant parameter, pressure, flow, level or temperature and supplying a signal that is proportional to the measured parameter. The output signal can then be processed by another instrument or equipment to provide indication, alarms or automatic control. There are a number of standard signals; however those common are the 1-5V and 4-20 mA electrical and electronic signal the 3-15 psi or 20-100 kPa pneumatic signal. I remember Engr. Salvaña always reminds us to remember this standards when we go and apply for a job. It helped though. Thanks to our hydraulics class instructor.
Output instrumentation includes devices such as solenoids, valves, regulators, circuit breakers, and relays. These devices control a desired output variable, and provide either remote or automated control capabilities. These are often referred to as final control elements when controlled remotely or by a control system.
Transmitters are devices which produce an output signal, often in the form of a 4–20 mA electrical current signal, although many other options using voltage, frequency, pressure, or ethernet are possible. This signal can be used for informational purposes, or it can be sent to a PLC, DCS, SCADA system, or other type of computerized controller, where it can be interpreted into readable values and used to control other devices and processes in the system.
Control Instrumentation plays a significant role in both gathering information from the field and changing the field parameters, and as such are a key part of control loops.