A more thorough explanation:

Accretion in law refers to the slow addition of land next to water due to the water’s movement or the increase of beneficiary’s share of a trust due to the actions of another beneficiary.

For example, accretion from natural causes (also referred to as alluvion) adds soil, sand, and other types of earth to the part of a person’s property that borders water. This occurs very slowly, but over time, a piece of property may grow a lot and increase its value. Any addition to one’s property by accretion becomes that person’s legal property. This is in contrast with avulsion where the original owner of the moved earth continues to own it. Avulsion occurs where large parts of a person's property suddenly move away due to natural events such as flooding.

Accretion in estates occurs where a beneficiary to a trust receives more than their original share of the trust. Often, this occurs because one of the other beneficiaries does not take their share or does not fulfill a requirement for them to receive their share of the trust.

These examples illustrate how accretion can occur in different contexts, but in each case, it involves a slow and gradual increase in something over time.