n. 1 the act or an instance of immersing; the process of being immersed. 2 baptism by immersing the whole person in water. 3 mental absorption. 4 Astron. the disappearance of a celestial body behind another or in its shadow. immersion heater an electric heater designed for direct immersion in a liquid to be heated, esp. as a fixture in a hot-water tank.
[ ME f. LL immersio (as IMMERSE) ]