The Latin designation for the so-called 'seas' on the surface of the Moon before modern telescopes showed the dark areas to be dry plains. Since 1959 spacecraft probes and landings (both manned and un-manned) have provided much detailed information but their origin is still perplexing to scientists.

The singular is Mare (e.g. Mare Imbrium), but there is a marked tendency to delatinize this terminology (e.g. Sea of Tranquility, Sea of Fertility etc).