One that controls: a controller, not an observer of events.
also comp·trol·ler An officer who audits accounts and supervises the financial affairs of a corporation or of a governmental body.
A regulating mechanism, as in a vehicle or electric device.
Usage Note: In the 15th century, the word controller developed the alternate spelling comptroller as a result of an association between the first part of the word, cont, and the etymologically unrelated word count and its variant compt. Although the historical pronunciation of comptroller would be the same as for controller, evidence suggests that the spelling pronunciations (kŏmp-trōˈlər) and (kŏmpˈtrōˌlər) may now be used by a majority of speakers. In a recent survey, 43 percent of the Usage Panel indicated that they pronounce comptroller like controller, while 57 percent pronounce it with mp, as it is spelled, with stress on either the first or second syllable. And half of those Panelists who pronounce comptroller like controller indicated that they also consider the spelling pronunciations acceptable.
(2) An electronic circuit board or system. In a computer, controllers contain the circuitry to run a peripheral device and are either contained on a plug-in expansion board or on the motherboard. In larger computers, a controller may be contained on one or more boards or in an external, stand-alone cabinet.