accretion

ot noun

1. The act of increasing, or the matter added, by an accession of parts externally; an extraneous addition A mineral ... augments not by growth, but by accretion.
2. Concretion; coherence of separate particles the accretion of particles so as to form a solid mass
3. The act of increasing by natural growth; especially the increase of organic bodies by the internal accession of parts; organic growth.
4. Something added externally to promote growth the external growth of an item.