heat ‧ er S3 /ˈhiːtə $ -ər/ BrE AmE noun [countable]
[ Word Family: noun : ↑ heat , ↑ heater , ↑ heating ; verb : ↑ heat , ↑ overheat ; adverb : ↑ heatedly ; adjective : ↑ heated ≠ UNHEATED ]
a machine for making air or water hotter:
Did you turn the heater off?