The Collaborative International Dictionary of English v.0.48:

Uti possidetis \U`ti pos`si*de"tis\ [L., as you possess.]
   [1913 Webster]
   1. (Internat. Law) The basis or principle of a treaty which
      leaves belligerents mutually in possession of what they
      have acquired by their arms during the war. --Brande & C.
      [1913 Webster]

   2. (Roman Law) A species of interdict granted to one who was
      in possession of an immovable thing, in order that he
      might be declared the legal possessor. --Burrill.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

UTI POSSIDETIS. This phrase, which means as you possess, is used in
international law to signify that the parties to a treaty are to retain
possession of what they have acquired by force during the war.