Full Definition of ACCRETION. 1. : 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. — ac·cre·tion·ary \-shə-ˌner-ē, -ˌne-rē\ adjective.