What is the difference between a controller and a compensator? I recently revisited some texts on control theory. In some textbooks the term compensator is used, seemingly interchangeably with controller (see e.g. The Control Handbook by Levine, chapter 14 on classical control system design methods).

This confuses me. In my field (aerospace engineering), we always talked about controllers. Since it's some time since my days at university I may have missed/forgotten some subtle differences...

Please help me to understand the difference:


What is the (technical) difference between a compensator and a controller? 
When do you call it one way or the other?
As the book's title is control theory; a controller is the general broad name given to any piece of hardware (or sowftware) that aims to control some processes, in your aerodynamics field probably the flight process; by menas of receiving signals from sensors and sending commands to propellers - actuators, and in between is the control hardware or software...

A compensator is a specific control-box-tool to correct (or compensate) some lacking aspect of a system property under concern, such as phase, lag, magnitude etc... It is generally useful as part of a bigger control system, rather than itself being a controller except some simple applications.

Obviously there are many different control system examples in addition to a compensator...