Definition
A piece of land entirely surrounded by water. In law, the term carries the same geographic meaning as in ordinary usage but acquires legal significance primarily through property law, admiralty, and the rules governing ownership of newly formed or emergent land.
The legal significance of an island depends largely on how and where it formed:
1. Islands in the open sea: Unowned islands arising in international waters belong to the first occupant (the nation that first takes possession and exercises sovereign control).
2. Islands within a state's territorial boundary: When an island forms within the territorial limits of a state — typically within a state's coastal waters or within a river forming a boundary — the island falls within that state's jurisdiction and ownership.
3. Islands arising in rivers: When a new island forms in a navigable or non-navigable river, ownership turns on the island's position relative to the river's thread (center line). Islands forming at the midpoint belong in equal shares to the riparian owners on each bank. Islands forming predominantly on one side of the thread belong to the riparian owner on that side.
Common Language
Modern common usage (Wiktionary): A tract of land surrounded by water. Also used colloquially as a shorthand for specific well-known islands (Long Island, Prince Edward Island, Epstein Island) and informally for certain prisons on islands.
Historical common usage (Webster's 1913): "A tract of land surrounded by water, and smaller than a continent." Webster's extended the term figuratively to any isolated mass resembling an island, such as an island of ice.
The legal definition tracks the geographic definition closely, but the law adds a critical layer: the geographic fact of being surrounded by water triggers specific rules about how an island is owned, who owns it, and what happens when water levels change. Common usage carries none of this operational weight. A layperson calls Long Island an island; a property lawyer asks whether it formed by accretion, avulsion, or artificial fill — and against what prior boundary.
Recognized Forms
/SUBTYPES
Oceanic island: Formed in open sea or international waters. Ownership determined by discovery and occupation under international law.
Territorial island: Formed within a state's coastal or inland waters. Ownership presumed in the state, subject to prior grants.
Riparian island: Formed within a river channel. Ownership allocated between adjacent riparian landowners based on position relative to the thalweg (the deepest channel line) or the thread of the stream.
Artificial island: Land created by human construction rather than natural forces. Generally does not receive the same common-law treatment as naturally formed islands; ownership typically vests in the party who created it, subject to applicable permits and sovereign rights.
Why It Matters in Research
The primary research trap with this term is conflating the physical description of land as an "island" with the legal conclusions that flow from how it formed and where it sits. Pre-twentieth-century cases and treatises make little distinction between riparian and coastal contexts, and researchers working with older materials must reconstruct that distinction from context.
Riparian island ownership was heavily litigated in the nineteenth century, particularly along the Mississippi, Ohio, and Great Lakes systems as settlement pushed west and timber and mineral rights became valuable. Cases often turn on minute questions of hydrographic surveying — whether the island formed on one side of the thalweg or another — and the factual record in older cases may be unreliable by modern standards.
The term also surfaces in boundary disputes between states. When a river forms a state boundary, an island's presence or formation can shift jurisdictional lines, and courts (including the U.S. Supreme Court) have addressed these questions in original jurisdiction proceedings. Researchers looking at state boundary disputes should not assume that the rules governing private riparian ownership apply without modification to sovereign boundary questions.
In admiralty and international law, the definition of an island matters for baseline and territorial sea calculations. UNCLOS Article 121 imposes specific requirements that an island be naturally formed land above water at high tide to generate the full maritime entitlements of a continental coast. Low-tide elevations and artificial structures are treated differently. This technical distinction is largely absent from common-law dictionary sources but is essential for any research touching maritime jurisdiction.
Historical Dictionary Support
The historical sources converge on the simple definition — a piece of land surrounded by water — without meaningful divergence. Black's first edition gives the bare geographic statement; the second edition adds two Michigan citations that put the definition in a riparian property context. Bouvier's entry, though partially garbled in the available text, contributes the substantive rule: open-sea islands go to the first occupant; islands forming near a coastline belong to the adjacent state; river islands forming at midstream divide equally between riparian owners, while islands forming closer to one bank belong to the nearer owner.
The historical sources are thin relative to the term's legal importance. Neither Black's edition provides the thalweg rule explicitly, and neither addresses the international law dimension or the distinction between natural and artificial formation. Researchers should not treat the dictionary definitions as complete statements of the applicable law; they are starting points that must be supplemented with treatises on water law and riparian rights (Gould on Waters, Kinney on Irrigation, Pomeroy on Riparian Rights) for any serious research question.
Jurisdictional Note
Western states following prior appropriation doctrines for water rights may apply different default rules to river islands than eastern states following riparian doctrine. Several western state constitutions — including those of Idaho, Montana, North Dakota, Washington, and Wyoming — contain specific provisions governing water and related land rights that can affect how emergent land is treated. Researchers should not assume common-law riparian island rules apply uniformly across jurisdictions.