An Etymological Dictionary of Astronomy and Astrophysics

An area on the surface of the → Moon that appears darker and smoother than its surroundings.
Once thought to be seas, lunar maria are now known to be basaltic basins created by volcanic → lava floods; plural maria.

See also: → lunar; L. mare “sea,” plural form maria, because
Galileo thought the dark featureless areas on the Moon were → seas.