The Collaborative International Dictionary of English v.0.48:

accretion \ac*cre"tion\ ([a^]k*kr[=e]"sh[u^]n), n. [L. accretio,
   fr. accrescere to increase. Cf. Crescent, Increase,
   Accrue.]
   [1913 Webster]
   1. The act of increasing by natural growth; esp. the increase
      of organic bodies by the internal accession of parts;
      organic growth. --Arbuthnot.
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   2. The act of increasing, or the matter added, by an
      accession of parts externally; an extraneous addition; as,
      an accretion of earth.
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            A mineral . . . augments not by growth, but by
            accretion.                            --Owen.
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            To strip off all the subordinate parts of his
            narrative as a later accretion.       --Sir G. C.
                                                  Lewis.
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   3. Concretion; coherence of separate particles; as, the
      accretion of particles so as to form a solid mass.
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   4. A growing together of parts naturally separate, as of the
      fingers or toes. --Dana.
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   5. (Law)
      (a) The adhering of property to something else, by which
          the owner of one thing becomes possessed of a right to
          another; generally, gain of land by the washing up of
          sand or soil from the sea or a river, or by a gradual
          recession of the water from the usual watermark.
      (b) Gain to an heir or legatee, by failure of a coheir to
          the same succession, or a co-legatee of the same
          thing, to take his share. --Wharton. Kent.
          [1913 Webster]

WordNet (r) 3.0 (2006):

accretion
    n 1: an increase by natural growth or addition [syn:
         accretion, accumulation]
    2: something contributing to growth or increase; "he scraped
       away the accretions of paint"; "the central city surrounded
       by recent accretions"
    3: (astronomy) the formation of a celestial object by the effect
       of gravity pulling together surrounding objects and gases
    4: (biology) growth by addition as by the adhesion of parts or
       particles
    5: (geology) an increase in land resulting from alluvial
       deposits or waterborne sediment
    6: (law) an increase in a beneficiary's share in an estate (as
       when a co-beneficiary dies or fails to meet some condition or
       rejects the inheritance)

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

ACCRETION. The increase of land by the washing of the seas or rivers. Hale,
De Jure Maris, 14. Vide Alluvion; Avulsion.