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]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "immersion":
   absorbed attention, absorption, affusion, application, aspergation,
   aspersion, baptism, baptismal gown, baptismal regeneration,
   baptistery, baptizement, bath, bathing, bedewing, burial, chrismal,
   christening, concentration, contemplation, dampening, damping,
   deep study, deep thought, deluge, dewing, dip, dipping, dousing,
   drowning, duck, ducking, dunking, engagement, engrossment,
   engulfment, flooding, font, hosing, hosing down, humidification,
   immergence, infusion, intentness, inundation, involvement,
   irrigation, laving, meditation, moistening, monomania, obsession,
   preoccupation, profound thought, rapt attention, rinsing,
   single-mindedness, sinking, souse, sousing, sparging, spattering,
   splashing, splattering, spraying, sprinkling, studiousness, study,
   submergence, submersion, swashing, total immersion, watering,
   wetting