1. The process in which separate parts or particles come together into a solid mass; such as, a hard lump produced by the concretion of mineral salts which are found in hollow organs or ducts of the body: "The doctor told him that there were concretions in his kidneys; which are commonly called, "kidney stones".
2. A hard solid mass formed by parts uniting into a whole; for example, an increase in the density of something.
3. A mass of inorganic material in a body organ or tissue, usually caused by a disease in the formation of stonelike objects within a body organ; for example, the kidneys.