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.
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   2. Submersion in water for the purpose of Christian baptism,
      as, practiced by the Baptists.
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   3. The state of being overhelmed or deeply absorbed; deep
      engagedness.
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            Too deep an immersion in the affairs of life.
                                                  --Atterbury.
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   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.
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   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.
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