Controller \Con*trol"ler\, n.

1. (Elec.) Any electric device for controlling a circuit or system; specif.:
   (a) An electromagnet, excited by the main current, for throwing a regulator magnet into or out of circuit in an automatic device for constant current regulation.
   (b) A kind of multiple switch for gradually admitting the current to, or shutting it off from, an electric motor; as, a car controller for an electric railway car.

2. (Mach.) A lever controlling the speed of an engine; -- applied esp. to the lever governing a throttle valve, as of a steam or gasoline engine, esp. on an automobile.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Controller \Con*trol"ler\, n. [From control, v. t.: cf. F. contr[^o]leur.]

1. One who, or that which, controls or restraines; one who has power or authority to regulate or control; one who governs.

The great controller of our fate Deigned to be man, and lived in low estate. --Dryden.

2. An officer appointed to keep a counter register of accounts, or to examine, rectify, or verify accounts. [More commonly written controller.]

3. (Naut.) An iron block, usually bolted to a ship's deck, for controlling the running out of a chain cable. The links of the cable tend to drop into hollows in the block, and thus hold fast until disengaged.

Source: Webster's Revised Unabridged Dictionary (1913)