The Collaborative International Dictionary of English v.0.48:

Accession \Ac*ces"sion\, n. [L. accessio, fr. accedere: cf. F.
   accession. See Accede.]
   1. A coming to; the act of acceding and becoming joined; as,
      a king's accession to a confederacy.
      [1913 Webster]

   2. Increase by something added; that which is added;
      augmentation from without; as, an accession of wealth or
      territory.
      [1913 Webster]

            The only accession which the Roman empire received
            was the province of Britain.          --Gibbon.
      [1913 Webster]

   3. (Law)
      (a) A mode of acquiring property, by which the owner of a
          corporeal substance which receives an addition by
          growth, or by labor, has a right to the part or thing
          added, or the improvement (provided the thing is not
          changed into a different species). Thus, the owner of
          a cow becomes the owner of her calf.
      (b) The act by which one power becomes party to
          engagements already in force between other powers.
          --Kent.
          [1913 Webster]

   4. The act of coming to or reaching a throne, an office, or
      dignity; as, the accession of the house of Stuart; --
      applied especially to the epoch of a new dynasty.
      [1913 Webster]

   5. (Med.) The invasion, approach, or commencement of a
      disease; a fit or paroxysm. AS
      [1913 Webster]

   Syn: Increase; addition; augmentation; enlargement.
        [1913 Webster]

WordNet (r) 3.0 (2006):

accession
    n 1: a process of increasing by addition (as to a collection or
         group); "the art collection grew through accession"
    2: (civil law) the right to all of that which your property
       produces whether by growth or improvement
    3: something added to what you already have; "the librarian
       shelved the new accessions"; "he was a new addition to the
       staff" [syn: accession, addition]
    4: agreeing with or consenting to (often unwillingly);
       "accession to such demands would set a dangerous precedent";
       "assenting to the Congressional determination" [syn:
       accession, assenting]
    5: the right to enter [syn: entree, access, accession,
       admission, admittance]
    6: the act of attaining or gaining access to a new office or
       right or position (especially the throne); "Elizabeth's
       accession in 1558" [syn: accession, rise to power]
    v 1: make a record of additions to a collection, such as a
         library

Moby Thesaurus II by Grady Ward, 1.0:

185 Moby Thesaurus words for "accession":
   accedence, acceptance, access, accessory, accompaniment, accretion,
   accrual, accruement, accumulation, acquiescence, acquirement,
   acquisition, addenda, addendum, additament, addition, additive,
   additory, additum, adjunct, adjunction, adjuvant, advance, advent,
   affixation, afflux, affluxion, agglutination, aggrandizement,
   agreement, agreement in principle, amplification, annex,
   annexation, anointing, anointment, appanage, appendage, appendant,
   appointment, appreciation, approach, approaching, appropinquation,
   approximation, appulse, appurtenance, appurtenant, arrogation,
   ascent, assent, assentation, assignment, assumption, attachment,
   attainment, augment, augmentation, authorization, ballooning,
   bloating, boom, boost, broadening, buildup, coda, coming,
   coming by, coming near, coming toward, complement, compliance,
   concomitant, concurrence, consecration, consent, continuation,
   corollary, coronation, crescendo, delegation, deputation,
   development, dragging down, earnings, edema, election, elevation,
   empowerment, enlargement, enthronement, expansion, extension,
   extrapolation, fixture, flood, flowing toward, forthcoming, gain,
   gaining, general agreement, getting, getting hold of, greatening,
   growth, gush, hearty assent, hike, imminence, inauguration,
   increase, increment, induction, inflation, installation,
   installment, instatement, investiture, joining, jump, junction,
   juxtaposition, leap, legitimate succession, making, moneygetting,
   moneygrubbing, moneymaking, mounting, multiplication, nearing,
   nearness, obtainment, obtention, offshoot, oncoming, pendant,
   placement, prefixation, procural, procurance, procuration,
   procurement, productiveness, proliferation, proximation, raise,
   reinforcement, rise, securement, seizure, side effect, side issue,
   snowballing, spread, succession, suffixation, superaddition,
   superfetation, superjunction, superposition, supplement,
   supplementation, support, surge, swelling, tailpiece,
   taking office, taking over, trover, tumescence, undergirding,
   uniting, up, upping, upsurge, upswing, uptrend, upturn, usurpation,
   warm assent, waxing, welcome, widening, winning



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ACCESSION, international law, is the absolute or conditional acceptance by
one or several states, of a treaty already concluded between one or several
states, of a treaty already concluded between other sovereignties. Merl. Rep.

mot Accession.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ACCESSION, property. The ownership of a thing, whether it be real or
personal, movable or immovable, carries with it the right to all that the
thing produces, and to all that becomes united to it, either naturally or
artificially; this is called the right of accession.
     2.-1. The doctrine of property arising from accession, is grounded on
the right of occupancy.
     3.-2. The original owner of any thing which receives an accession by
natural  or artificial means, as by the growth of vegetables, the pregnancy
of animals; Louis. Code, art. 491; the embroidering of cloth, or the
conversion of wood or metal into vessels or utensils, is entitled to his
right of possession to the property of it, under such its state of
improvement; 5 H. 7, 15; 12 H. 8, 10; Bro. Ab. Propertie, 23; Moor, 20;
Poph. 88.  But the owner must be able to prove the identity of the original
materials; for if wine, oil, or bread, be made out of another man's grapes,
olives, or wheat, they belong to the new operator, who is bound to make
satisfaction to the former proprietor for the materials which he has so
converted. 2 Bl. Com. 404; 5 Johns. Rep. 348; Betts v. Lee, 6 Johns. Rep.
169; Curtiss v. Groat, 10 Johns. 288; Babcock v. Gill, 9 Johns. Rep. 363;
Chandler v. Edson, 5 H. 7, 15; 12 H. 8, 10; Fits. Abr. Bar. 144; Bro. Abr.
Property, 23; Doddridge Eng. Lawyer, 125, 126, 132, 134. See Adjunction;
Confusion of Goods. See Generally, Louis. Code, tit. 2, c. 2 and 3.