A mere is a body of water hemmed by land. Most meres are freshwater, and most lie in the northern half of Earth. Great meres are sometimes called 'inland seas' and small seas are sometimes called meres.
Finland is known as The Land of the Thousand Meres (in truth there are 187,888 meres in Finland, of which 60,000 are great) and Maniwaters is known as The Land of Ten Thousand Meres.