The Collaborative International Dictionary of English v.0.48:

Concretion \Con*cre"tion\, n. [L. concretio.]
   1. The process of concreting; the process of uniting or of
      becoming united, as particles of matter into a mass;
      solidification.
      [1913 Webster]

   2. A mass or nodule of solid matter formed by growing
      together, by congelation, condensation, coagulation,
      induration, etc.; a clot; a lump; a calculus.
      [1913 Webster]

            Accidental ossifications or deposits of phosphates
            of lime in certain organs . . . are called osseous
            concretions.                          --Dunglison.
      [1913 Webster]

   3. (Geol.) A rounded mass or nodule produced by an
      aggregation of the material around a center; as, the
      calcareous concretions common in beds of clay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

concretion
    n 1: the formation of stonelike objects within a body organ
         (e.g., the kidneys)
    2: a hard lump produced by the concretion of mineral salts;
       found in hollow organs or ducts of the body; "renal calculi
       can be very painful" [syn: calculus, concretion]
    3: an increase in the density of something [syn: compaction,
       compression, concretion, densification]
    4: the union of diverse things into one body or form or group;
       the growing together of parts [syn: coalescence,
       coalescency, coalition, concretion, conglutination]