1. The growth or increase in size by gradual external addition, accumulation, fusion, inclusion, or the growing together of separate things.

People who have money in the bank may expect interest to accrue or to grow as the years pass; so there are those who may then refer of the accretion of their capital.

2. In astronomy, a process in which matter that is revolving around an astronomical object is gradually pulled in and added to the body's mass as a result of gravity.
3. A process by which a body of rock or a landmass increases in size as a result of material accumulating on or around it resulting from alluvial deposits or water-borne sediments.