[Editor's note: This Q&A from Environment Canada explains the rough difference between types of hydrological features. Names in the real world are often messier than this text book explanation. One way we've tried to help in Natural Earth is by indicating if a lake is freshwater, saline, natural, artificial, stable water level, seasonal water level, or simply ephemeral.]
Republished from Environment Canada. Feb. 2002.
What is the difference between a sea and a lake? Looking at the names of many sea and many lakes does not readily demonstrate an identifiable difference. There are salt water lakes and fresh water seas and some lakes that are bigger than other seas.
In order to understand why some smaller salt water bodies are called lakes and others seas it is necessary to realize that lakes are, in geological time scales, transitory in nature–they form, mature and die.
Some water bodies that started out as saltwater seas over time became closed-off from the oceans. Depending on the quantity of fresh water flowing in from rivers, glacial melt water, or other sources, the salinity could have declined to the point where the water became relatively fresh. The Sea of Aral is probably an example of where this transition has occurred. Similarly the reverse can occur whereby freshwater lakes can become open to the sea so that the salinity increases, as in the Baltic Sea. The Black Sea is an example that has alternated between fresh and salt water conditions over geological time. Evidence for these changes can be found in ancient fossils of organisms some of which were known to be tolerant of saltwater while others were known to have been intolerant.
No doubt there was also some confusion in the naming of water bodies by the early explorers based on their first impressions and certainly one can understand why some may have been inappropriately named. In addition the subsequent translation of the names between different languages could also have added to the confusion.
Here are some definitions of water bodies: