The Collaborative International Dictionary of English v.0.48:

Inclusion \In*clu"sion\, n. [L. inclusio: cf. F. inclusion. See
   Include.]
   [1913 Webster]
   1. The act of including, or the state of being included;
      limitation; restriction; as, the lines of inclusion of his
      policy. --Sir W. Temple.
      [1913 Webster]

   2. Something that is included.
      [PJC]

   3. (Min.) A foreign substance, either liquid or solid,
      usually of minute size, inclosed in the mass of a mineral.
      [1913 Webster]

   4. (Biol., Cytology) A small body suspended within the
      cytoplasm of a cell.
      [PJC]

   5. (Logic, Math.) The relationship existing between two sets
      if one is a subset of the other.
      [PJC]

WordNet (r) 3.0 (2006):

inclusion
    n 1: the state of being included [ant: exclusion]
    2: the relation of comprising something; "he admired the
       inclusion of so many ideas in such a short work" [syn:
       inclusion, comprehension]
    3: any small intracellular body found within another
       (characteristic of certain diseases); "an inclusion in the
       cytoplasm of the cell" [syn: inclusion body, cellular
       inclusion, inclusion]
    4: the act of including

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "inclusion":
   Anschluss, absorption, addition, affiliation, agglomeration,
   aggregation, agreement, alignment, alliance, amalgamation,
   assimilation, association, beleaguerment, besetment, blend,
   blending, blockade, blockading, cabal, cahoots, cartel,
   centralization, cincture, circling, circumambience, circumambiency,
   circumcincture, circumflexion, circumjacence, circumposition,
   circumscription, classification, coadunation, coalescence,
   coalition, colleagueship, collegialism, collegiality, combination,
   combine, combo, composition, comradeship, confederacy,
   confederation, confinement, confraternity, congeries,
   conglomeration, conjugation, conjunction, consolidation,
   conspiracy, containment, copartnership, copartnery, cordoning,
   counting, ecumenism, embarrassment, embodiment, embracement,
   encincture, encirclement, enclosure, encompassment, enfoldment,
   engagement, enmeshment, enosis, entanglement, envelopment,
   environment, federalization, federation, fellowship, fraternalism,
   fraternity, fraternization, freemasonry, fusion, girding, girdling,
   grouping, hookup, immurement, implication, imprisonment,
   incarceration, incorporation, integration, involution, involvement,
   junction, junta, league, marriage, meld, melding, merger,
   numbering, package, package deal, partnership, quarantine,
   relation, siege, sodality, solidification, sorority, surrounding,
   syncretism, syndication, syneresis, synthesis, tie-in, tie-up,
   unification, union, wedding