Terminology:
The manipulated variable (MV) is a measure of resource being fed into the process, for instance how much thermal energy.
A final control element (FCE) is the device that changes the value of the manipulated variable.
The controller output (CO) is the signal from the controller to the final control element.
The process variable (PV) is a measure of the process output that changes in response to changes in the manipulated variable.
The set point (SP) is the value at which we wish to maintain the process variable at.