The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            The great controller of our fate
            Deigned to be man, and lived in low estate.
                                                  --Dryden.
      [1913 Webster]

   2. An officer appointed to keep a counter register of
      accounts, or to examine, rectify, or verify accounts.
      [More commonly written controller.]
      [1913 Webster]

   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.
      [1913 Webster]

   4. (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.
          [Webster 1913 Suppl.]

   5. (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.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

controller
    n 1: someone who maintains and audits business accounts [syn:
         accountant, comptroller, controller]
    2: a person who directs and restrains [syn: restrainer,
       controller]
    3: a mechanism that controls the operation of a machine; "the
       speed controller on his turntable was not working properly";
       "I turned the controls over to her" [syn: control,
       controller]

The Free On-line Dictionary of Computing (19 January 2023):

controller

    Part of a computer, typically a separate circuit
   board, which allows the computer to use certain kinds of
   peripheral devices.  A disk controller is used to connect
   hard disks and floppy disks, a network controller is
   used for Ethernet.  Other controllers are: keyboard
   controller, interrupt controller and graphics controller.

   (1998-03-16)