What is the X in X-mas?
mid-15c., from Late Latin immersionem (nominative immersio), noun of action from past participle stem of immergere, from assimilated form of in- "into, in, on, upon" (see in- (2)) + Latin mergere "plunge, dip" (see merge). Meaning "absorption in some interest or situation" is from 1640s. As a method of teaching a foreign language, it is from 1965, trademarked by the Berlitz company.
immersion im·mer·sion (ĭ-mûr'zhən, -shən)
The placing of a body under water or other liquid.
The use of a fluid on a microscope slide in order to exclude air from between the glass slide and the bottom lens.