Driver

n. 1. The main loop of an event-processing program; the code that gets commands and dispatches them for execution.

2. [techspeak] In 'device driver', code designed to handle a particular peripheral device such as a magnetic disk, tape unit, etc.

3. In the TeX general, 'driver' also means a program that translates some device-independent or other common format to something a real device can actually understand.