The noun "immersion" refers to the act of immersing or being immersed, particularly in water for baptism or in deep engagement with something.

(Britain, Ireland, informal) An immersion heater.

(mathematics) A smooth map whose differential is everywhere injective, related to the mathematical concept of an embedding.

(astronomy) The disappearance of a celestial body, by passing either behind another, as in the occultation of a star, or into its shadow, as in the eclipse of a satellite.

(education) A form of foreign-language teaching where the language is used intensively to teach other subjects to a student.

One's suspension of disbelief while reading, playing a video game, etc. The experience of losing oneself in a fictional world.