The Collaborative International Dictionary
Mare clausum \Ma"re clau"sum\ [L.] (Internat. Law) Lit., closed sea; hence, a body of water within the separate jurisdiction of the nation; -- opposed to open sea ( mare liberum), the water open to all nations and over which no single nation has special control.
WordNet
n. (closed sea) a navigable body of water under the jurisdiction of a single nation
Wikipedia
Mare clausum ( legal Latin meaning "closed sea") is a term used in international law to mention a sea, ocean or other navigable body of water under the jurisdiction of a state that is closed or not accessible to other states. Mare clausum is an exception to mare liberum (Latin for "free sea"), meaning a sea that is open to navigation to ships of all nations. In the generally accepted principle of International waters, oceans, seas, and waters outside national jurisdiction are open to navigation by all and referred to as "high seas" or mare liberum. Portugal and Spain defended a Mare clausum policy during the age of discovery. This was soon challenged by other European nations.
Usage examples of "mare clausum".
The stormy ocean we behold in the west, which corresponds to our Atlantic, though it is far more of a mare clausum in the geographical sense, is also destined to become a calm and placid inland sea.