Source : Webster's Revised Unabridged Dictionary (1913)

Accretion \Ac*cre"tion\, n. [L. accretio, fr. accrescere to
   increase. Cf. {Crescent}, {Increase}, {Accrue}.]
   1. The act of increasing by natural growth; esp. the increase
      of organic bodies by the internal accession of parts;
      organic growth. --Arbuthnot.

   2. The act of increasing, or the matter added, by an
      accession of parts externally; an extraneous addition; as,
      an accretion of earth.

            A mineral . . . augments not by grown, but by
            accretion.                            --Owen.

            To strip off all the subordinate parts of his as a
            later accretion.                      --Sir G. C.
                                                  Lewis.

   3. Concretion; coherence of separate particles; as, the
      accretion of particles so as to form a solid mass.

   4. A growing together of parts naturally separate, as of the
      fingers toes. --Dana.

   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 sail 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, failure of a coheir to the
          same succession, or a co-legatee of the same thing, to
          take his share. --Wharton. Kent.

Source : WordNet®

accretion
     n 1: an increase by natural growth or addition [syn: {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 water-borne 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)