The Collaborative International Dictionary of English v.0.48:

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.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

mare clausum
    n 1: (closed sea) a navigable body of water under the
         jurisdiction of a single nation