Source : Webster's Revised Unabridged Dictionary (1913)

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}, the
   water open to all nations and over which no single nation has
   special control.

Source : WordNet®

mare clausum
     n : (closed sea) a navigable body of water under the
         jurisdiction of a single nation