The Collaborative International Dictionary of English v.0.48:

Intromission \In`tro*mis"sion\, n. [Cf. F. intromission. See
   Intromit.]
   [1913 Webster]
   1. The act of sending in or of putting in; insertion.
      [1913 Webster]

   2. Specifically; (Zool.) The insertion of the male copulatory
      organ into the female in the process of coitus. --South.
      [1913 Webster +PJC]

            Four populations [of the vlei rat] varied in a
            number of parameters of copulatory behavior, such as
            latency to first mount, number of intromissions per
            series, and latency to intromission after first
            ejaculation.                          --Edith
                                                  Dempster
                                                  (African Small
                                                  Mammals
                                                  Newsletter,
                                                  Issue No. 16,
                                                  May 1996,
                                                  Laboratoir
                                                  Mammif[`e]res
                                                  & Oiseaux,
                                                  Paris)
      [PJC]

   3. The act of letting go in; admission.
      [1913 Webster]

   4. (Scots Law) An intermeddling with the affairs of another,
      either on legal grounds or without authority.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intromission
    n 1: the act of putting one thing into another [syn:
         insertion, introduction, intromission]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "intromission":
   acceptance, admission, admittance, baptism, embedment, enlistment,
   enrollment, entrance, graft, grafting, immission, impaction,
   impactment, implantation, inauguration, induction, infixion,
   infusion, initiation, injection, inoculation, insertion,
   insinuation, installation, instatement, interjection,
   interpolation, introduction, investiture, ordination, penetration,
   perfusion, tessellation, transplant, transplantation