A controller is
Under the EU Directive on the Protection of Personal Data, a controller (also called a data controller) is
|“||the natural or legal person, public authority, agency or any other body which alone or jointly with others determines the purposes and means of the processing of personal data; where the purposes and means of processing are determined by national or Community laws or regulations, the controller or the specific criteria for his nomination may be designated by national or Community law.||”|
A controller is "[a]nything that has the capability to affect a physical entity, like changing its state or moving it."
See Videogame controller.