lacus

Lacus is a Latin noun meaning lake or pool. In English, the term appears mainly in scholarly or geographic names and in the specialized language of planetary geology and astronomy. It is especially prominent in lunar nomenclature, where Lacus denotes a class of small plains on the Moon.

In lunar use, Lacus is an official IAU designation for a small plain, typically formed by ancient lava flows and smaller than a mare. These features are named with descriptive Latin phrases or adjectives rather than mythological names. They occur across the lunar surface and can range from tens to a few hundred kilometers across. Notable examples include Lacus Mortis, also translated as the Lake of Death, and Lacus Somniorum, the Lake of Dreams.

Outside of lunar nomenclature, lacus simply means a lake in classical Latin and appears in historical texts and modern scientific names that borrow Latin vocabulary. In English usage, the ordinary term for a standing body of water remains “lake,” with lacus used primarily in Latin phrases or when referring to the Moon’s geography.