The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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]