Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

            The only accession which the Roman empire received
            was the province of Britain.          --Gibbon.

   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.

   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.

   5. (Med.) The invasion, approach, or commencement of a
      disease; a fit or paroxysm.

   Syn: Increase; addition; augmentation; enlargement.

Source : WordNet®

accession
     n 1: a process of increasing by addition (as to a collection or
          group); "the art collectin 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: {addition}]
     4: agreeing with or consenting to (often unwillingly);
        "accession to such demands would set a dangerous
        precedent"; "assenting to the Congressional determination"
        [syn: {assenting}]
     5: the right to enter [syn: {entree}, {access}, {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: {rise to power}]
     v : make a record of additions to a collection, such as a
         library