From Longman Dictionary of Contemporary Englishcar alarmˈcar aˌlarm noun [countable] TTCequipment in a car that makes a loud noise if anyone tries to steal or damage the car
Examples from the Corpus
car alarm• In my San Francisco neighborhood, some one has a car alarm.• Have you ever noticed the response when a car alarm goes off in a busy street?• Down among the orange lamps a broken car alarm marked out the time.• She breaks the window of the black car - and the car alarm goes off!• He set the car alarm, and went up to the door. 