From The Collaborative International Dictionary of English v.0.48 [gcide]:
Immersion \Im*mer"sion\, n. [L. immersio; cf. F. immersion.]
1. The act of immersing, or the state of being immersed; a
sinking within a fluid; a dipping; as, the immersion of
Achilles in the Styx.
[1913 Webster]
2. Submersion in water for the purpose of Christian baptism,
as, practiced by the Baptists.
[1913 Webster]
3. The state of being overhelmed or deeply absorbed; deep
engagedness.
[1913 Webster]
Too deep an immersion in the affairs of life.
--Atterbury.
[1913 Webster]
4. (Astron.) The dissapearance of a celestail body, by
passing either behind another, as in the occultation of a
star, or into its shadow, as in the eclipse of a
satellite; -- opposed to emersion.
[1913 Webster]
Immersion lens, a microscopic objective of short focal
distance designed to work with a drop of liquid, as oil,
between the front lens and the slide, so that this lens is
practically immersed.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
immersion
n 1: sinking until covered completely with water [syn:
submergence, submerging, submersion, immersion]
2: (astronomy) the disappearance of a celestial body prior to an
eclipse [syn: ingress, immersion] [ant: egress,
emersion]
3: complete attention; intense mental effort [syn:
concentration, engrossment, absorption, immersion]
4: a form of baptism in which part or all of a person's body is
submerged
5: the act of wetting something by submerging it [syn:
submersion, immersion, ducking, dousing]