Latin: The sea. Mare clausum. The sea closed; that is, not open or free. The title of Selden’s great work, intended as an answer to the Mare Lib erum of Grotius; in which he undertakes to prove the sea to be capable of private dominion. 1 Kent, Comm. 27.Mare liberum. The sea free. The title of a work written by Grotius against the Portuguese claim to an exclusive trade to the Indies, through the South Atlantic and Indian oceans; showing that the sea was not capable of private dominion. 1 Kent, Comm. 27.