Literally lake, a term used in the names of certain dark, isolated features on the Moon. Its use dates from a time when it was believed that the darker features on the Moon were of liquid water; this is now known to be untrue. In recognition of its use over a long period, the term has been retained in official place names on the Moon. Lacus was also formerly used in the names of certain features on Mars. Though these names are still often used by amateur astronomers, they have been replaced officially by more precise descriptive terms. Thus, what was known as Solis Lacus is now properly called Solis Planum.