What is Adverse Possession?

Legal Definition
Adverse possession is a method of acquiring title to real property by possession for a statutory period under certain conditions, viz: proof of non-permissive use which is actual, open and notorious, exclusive, adverse, and continuous for the statutory period. It is governed by statute concerning the title to real property (land and the fixed structures built upon it). By adverse possession, title to another's real property can be acquired without compensation, by holding the property in a manner that conflicts with the true owner's rights for a specified period. For example, squatter's rights are a specific form of adverse possession.

The circumstances in which adverse possession arises determine the type of title acquired by the disseisor (the one who obtains the title from the original owner), which may be fee simple title, mineral rights, or another interest in real property. Adverse possession's origins are based both in statutory actions and in common law precepts, so the details concerning adverse possession actions vary by jurisdiction. The required period of uninterrupted possession is governed by the statute of limitations. Other elements of adverse possession are judicial constructs.

Chattel property may also be adversely possessed, but owing to the vast differences between real and chattel property, the rules respecting such attempt are rather more stringent, favoring the original owner rather than the adverse possessor/disseisor; such rules find particular application respecting works of art.
-- Wikipedia
Legal Definition
Adverse possession is a doctrine under which a person in possession of land owned by someone else may acquire valid title to it, so long as certain common law requirements are met, and the adverse possessor is in possession for a sufficient period of time, as defined by a statute of limitations.

The common law requirements

The common law requirements have evolved over time, and the articulation of those requirements varies somewhat from jurisdiction to jurisdiction. Typically, adverse possession, in order to ripen into title, must be:

(1) Continuous; this means continual.

(2) Hostile to the interests of the true owner; this is the adverse part of adverse possession.

(3) Open and notorious, so as to put the true owner on notice that a trespasser is in possession.

(4) Actual, so that the true owner has a cause of action for trespass, on which the true owner must act within the statute of limitations.

(5) Exclusive, in order that there be no confusion as to who acquires title once the time has run.

The statute of limitations

A typical statute will require possession for 7 years, if under color of title, or 20 years, if not.

A mnemonic may help with remembering the decisional and statutory elements of adverse possession; think of it as inchoate ownership which becomes choaTe [(i.e. continuous, hostile, open, actual, for the requisite period of Time, and exclusive). Decisional pieces are indicated in lowercase, statutory ones in uppercase.].
Legal Definition
When a party claims ownership of a propety they have been in for more than 12 years. It can also be claimed on abandoned property or it can go unchallenged by the actual owner. Refer to color of title.
Legal Definition
Title to lands. The enjoyment of land, or such estate as lies in grant, under such circumstances as indicate that such enjoyment has been commenced and contiuued, under an assertion or color of right on the part of the possessor. 3 East, R. 394; 1 Pick. Rep. 466; 1 Dall. R. 67; 2 Serg. & Rawle, 527; 10 Watts R, 289; 8 Con R. 440; 3 Penn. 132; 2 Aik. 364; 2 Watts, 23; 9, John. 174; 18 John. 40, 355; 5 Pet. 402; 4 Bibb, 550. Actual possession is a pedis possessio which can be only of ground enclosed, and only such possession can a wrongdoer have. He can have no constructive possession. 7 Serg. & R. 192; 3 Id. 517; 2 Wash. C. Rep. 478, 479.

2. When the possession or enjoyment has been adverse for twenty years, of which the jury are to judge from the circumstances the law raises the presumption of a grant. Ang. on Wat. Courses, 85, et seq. But this presumption arises only when the use or occupation would otherwise have been unlawful. 3 Greenl. R. 120; 6 Binn. R. 416; 6 Cowen, R. 617, 677; Cowen, R. 589; 4 S. & R. 456. See 2 Smith's Lead. Cas. 307-416.

3. There are four general rules by which it may be ascertained that possession is not adverse; these will be separately considered.

4. – 1. When both parties claim under the same title; as, if a man seised of certain land in fee, have issue two sons and die seised, and one of the sons enter by abatement into the land, the statute, of limitations will not operate against the other son; for when the abator entered into the land of his father, before entry made by his brother, the law intends that he entered claiming as heir to his father, by which title the other son also claims. Co. Litt s. 396.

5. – 2. When the possession of the one party is consistent with the title of the other; as, where, the rents of a trust state were received by a cestui que trust for more than twenty years after the creation of the trust, without any interference, of the trustee, such ppssession being consistent with and secured to the cestui qwe trust by the terms of the deed, the receipt was held not to be adverse to the title of the trustee. 8 East. 248.

6. – 3. When, in contemplation of law, the claimant has never been out of possession; as, where Paul devised lands to John and his heirs, and died, and John died, and afterwards the heirs of John and a stranger entered, and took the profits for twenty years; upon ejectment brought by the devisee of the heir of John against the stranger, it was held that the perception of the rents and profits by the stranger was not adverse to the devisee's title; for when two men are in possession, the law adjudges it to be the possession of him who has the right. Lord Raym. 329.

7. – 4. When the occupier has acknowledged the claimant's titles; as, if a lease be granted for a term, and, after paying the rent for the land during such term, the tenant hold for twenty years without paying rent, his possession will not be adverse. See Bos. & P. 542; 8 B. & Cr. 717; 2 Bouv. Inst. n. 2193-94, 2351.
-- Bouviers Law Dictionary
Legal Definition
The actual, open, and notorious possession and enjoyment of reni property, or of any estate lying in grant, continued for a certam length of time, held adversely and in denial and opposition to the title of another claimant, or under circumstances which indicate an assertion or color of right or title on the part of the person maintaining it, as against another person who is out of possession. Costello v. Edson, 44 Minn. 135, 46 N. W. 299; Taylor v. Philippi, 35 W. Va. 554, 14 S. E. 130; Pickett v. Pope, 74 Ala. 122; Martin v. Maine Cent. R. Co.., 83 Me. i00, 21 Atl. 740; Dixon v. Co.ok, 47 Miss. 220.
-- Black's Law Dictionary
Legal Definition
Open and notorious possession and occupation of real property under an evident claim or color of right. See 35 L. R. A. (N. S.) 755, note.
-- Ballentine's Law Dictionary