accretion

noun
ac·​cre·​tion | \ ə-ˈkrē-shən How to pronounce accretion (audio) \

Definition of accretion

1 : the process of growth or enlargement by a gradual buildup: such 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