Source : Webster's Revised Unabridged Dictionary (1913)

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.

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 : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)