A written contract, by which the insurers oblige themselves to indemnify sea-risks under various conditions. An interest policy, is where the insurer has a real assignable interest in the thing insured; a wager policy, is where the insurer has no substantial interest in the thing insured; an open policy, is where the amount of interest is not fixed, but left to be ascertained in case of loss; a valued policy, is where an actual value has been set on the ship or goods.