A sea is generally defined as a large body of salt water that is partly or fully enclosed by land but also has an outlet to the ocean. A lake, on the other hand, is a body of freshwater that is completely surrounded by land, with no direct access to the ocean.

Despite their massive size and "ocean-like" horizons, the Great Lakes of North America are not considered seas because they fail two primary scientific criteria: salinity and connectivity. Geologically, a "sea" is typically a large body of saltwater that is either part of an ocean or has a direct, sea-level connection to it (like the Mediterranean). The Great Lakes are almost entirely freshwater, formed by glacial meltwater rather than oceanic flooding. Furthermore, they are landlocked and elevated; they sit hundreds of feet above sea level (Lake Superior is 600 feet up). While they are connected to the Atlantic via the St. Lawrence River, this is a downstream river connection involving numerous locks and rapids, not a tidal, sea-level exchange. While they are often called "Inland Seas" in poetry and marketing due to their 10,000 miles of coastline and massive storms, in the world of geography and limnology, they remain the largest freshwater lake system on Earth.