1. CONTROLLED VARIABLE: Process output variable that is maintained between specified limits.
2. MEASURED VALUE: Actual value of the controlled variable, as determined by the instrumentation.
3. DESIRED VALUE: Required value of the controlled variable (set point).
4. ERROR : The difference between the desired value and the measured value.
5. MANIPULATED VARIABLE: Process input variable that is adjusted, to steer the controlled variable towards the desired value.
6. DISTURBANCE VARIABLE: Process input variable that can cause the controlled variable to deviate from the desired value.