In public international law, accretion is one of the modes by which a state can acquire new territory.
It refers to the gradual increase of land on a state's territory through natural causes.
This can happen, for example, when a river slowly deposits silt along its bank (a process called alluvion) or when a new island is formed within a state's territorial waters due to volcanic activity.
The key element is that the process is natural and gradual, not man-made or a result of war or treaty.