noun /ʌˈkɹiː.ʃən/
a) The act of increasing by natural growth; especially the increase of organic bodies by the internal accession of parts; organic growth.
There might have been a slight accretion of the moss and lichen on the shingled roof.
b) The act of increasing, or the matter added, by an accession of parts externally; an extraneous addition; as, an accretion of earth.
A mineral ... augments not by grown, but by accretion.
See Also: accretion disk, accrete
Wikipedia foundation.