the process of growth or enlargement by a gradual buildup: as a : increase by external addition or accumulation (as by adhesion of external parts or particles) b : the increase of land by the action of natural forces 2 : a product of accretion; especially : an extraneous addition 〈accretions of grime〉 — ac•cre•tion•ary \-shə-ˌner-ē\ adjective — ac•cre•tive \ə-ˈkrē-tiv\ adjective