Immersion is the act of dipping something in a substance, completely covering it. It might be something physical, such as plunging your body into water, or metaphorical, such as becoming totally immersed in a project.
The Latin background of immersion includes the root mergere, "to plunge." The idea of immersion as a physical submergence came first and the idea of something being absorbed in a situation came about later. The idea behind immersion is that of complete coverage, whether that be by water, as in Baptism, or by attention, as in "immersion learning," an idea for learning a language trademarked in 1965 by the Berlitz company.