What does accretion mean?
The definition of accretion
Ac*cre”tion, n.
Etym: [L. accretio, fr. accrescere to increase. Cf. Crescent, Increase, Accrue.]

1 The act of increasing by natural growth; esp. the increase of organic bodies by the internal accession of parts; organic growth. Arbuthnot.
2 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. Owen. To strip off all the subordinate parts of his as a later accretion. Sir G. C. Lewis.
3 Concretion; coherence of separate particles; as, the accretion of particles so as to form a solid mass.
4 A growing together of parts naturally separate, as of the fingers toes. Dana.
5 (Law)

(a) The adhering of property to something else, by which the owner of one thing becomes possessed of a right to another; generally, gain of land by the washing up of sand or sail from the sea or a river, or by a gradual recession of the water from the usual watermark.
(b) Gain to an heir or legatee, failure of a coheir to the same succession, or a co-legatee of the same thing, to take his share. Wharton. Kent.