1. [n]  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"
2. [n]  someone who maintains and audits business accounts
3. [n]  a person who directs and restrains

1. \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.
   

2. \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.
   

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.