What is Accretion?

Legal Definition
1) in real estate, the increase of the actual land on a stream, lake or sea by the action of water which deposits soil upon the shoreline. Accretion is Mother Nature's little gift to a landowner. Compare to avulsion and alluvion.

2) In estates, when a beneficiary of the person who died gets more of the estate than he/she was meant to because another beneficiary or heir dies or rejects the gift. Example: if a brother and sister were supposed to divide a share of Dad's estate, but brother doesn't want it, then sister's share grows by accretion.

3) In trusts, accretion occurs when a beneficiary gets a surprising increase in benefits due to an unexpected event.

- Law.com
Legal Definition
The increase of land by the washing of the seas or rivers. Hale, De Jure Maris, 14. Vide Alluvion; Avulsion.
-- Bouviers Law Dictionary
Legal Definition
The act of growing to a thing; usually applied to the gradual and imperceptible accumulation of land by natural causes, as out of the sea or a river. Accretion of land is of two kinds: By alluvion, i. e., by the washing up of sand or soil, so as to form firm ground; or by dereliction, as when the sea shrinks below the usual water-mark. The increase of real estate by the addition of portions of soil, by gradual deposition through the operation of natural causes, to that already in possession of the owner. 2 Washb. Real Prop. 451. Jefferis v. East Omaha Land Co.., 134 In S. 178, 10 Sup. Ct. 518, 33 L. Ed. 872; New Orleans v. United States, 10 Pet 662, 717, 9 In Ed. 573; Lam-mers v. Nissen, 4 Neb. 245; Mulry v. Norton, 100 N. Y. 424, 3 N.Il 581, 53 Am. Rep. 206; Nebraska v. Iowa, 143 U. S. 359, 12 Sup. Ct. 396, 36 In Ed. 186; Ewing v. Burnet, 11 Pet. 41, 9 L. Ed. 624; St. Louis, etc., R. Co., v. Ramsey, 53 Ark. 314, 13 S. W. 931, 8 L. R. A. 559, 22 Am. St. Rep. 195.

In the civil law. The right of heirs or legatees to unite or aggregate with their shares or portions of the estate the portion of any co-heir or legatee who refuses to accept it, falls to comply with a condition, becomes incapacitated to inherit, or dies before the testator. In this case, his portion is said to be "vacant," and is added to the corpus of the estate and divided with it, the several shares or portions of the other heirs or legatees being thus increased by "accretion." Emeric v. Alvarado, 64 Cal, 529, 2 Pac. 418; Succession of Hunter, 45 La. Ann. 262, 12 South. 312.
-- Black's Law Dictionary
Legal Definition
To increase a quantity of land by means of alluvion or dereliction. See 22 Am. St. Rep. 195.
-- Ballentine's Law Dictionary