Adverse Possession
Definition
Adverse possession is a doctrine by which a person who occupies land belonging to another may acquire legal title if their possession is actual, exclusive, open and notorious, adverse/hostile (without permission), and continuous for the statutory period. The rationale includes encouraging productive use of land, settling disputes, and rewarding those who treat land as their own. The required state of mind varies by jurisdiction — some require good faith, others bad faith, and still others are indifferent to the possessor's subjective intent.
Example
A neighbor mistakenly builds a fence three feet onto another's property and uses the enclosed area openly for 20 years. If the statutory period has run, the neighbor may acquire title by adverse possession.