Controller

noun

1. One who controls something.
2. Any electric or mechanical device for controlling a circuit or system.
3. A person who audits, and manages the financial affairs of a company or government; a comptroller.
4. A mechanism that controls or regulates the operation of a machine, especially a peripheral device in a computer.
5. 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.