immersion heater

WordReference Random House Unabridged Dictionary of American English © 2025

immer′sion heat′er, 

1. a small electric coil used to heat a liquid, as a cup of water, in which it is immersed. Also called immer′sion coil′. 

• 1910–15

Collins Concise English Dictionary © HarperCollins Publishers::

immersion heater noun

1. an electrical device, usually thermostatically controlled, for heating the liquid in which it is immersed, esp as a fixture in a domestic hot-water tank