Longman Dictionary of Contemporary English
Wiktionary
n. A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water
WordNet
n. a heating element that is immersed in the liquid that is to be heated (as in a hot-water tank)
Usage examples of "immersion heater".
Frau Hufnagel doesn't grasp anything at all, but does the right thing: presuming the intruder in her fragile Christmas room to be a slave laborer from the East, now masterless, she holds out, with bravely trembling hands, the brand-new immersion heater, expecting the brute to blow off steam by demolishing this household utensil.
Now I remembered giving the cigarettes to the plumber who fixed the immersion heater.
While she was there she switched on the electric immersion heater, thinking how, piece by piece, a home was brought back to life.