月海是月球上大块呈黑色的玄武岩平原，推测是古代火山爆发的产物。

所谓的月海，并非月球上面的海洋。其之所以被称之为“海”，是因为早期的观察者，发现到月面有部分地区较暗。而在当时无法清晰观察到月球表面的情况下，观察者们按照其对地球的认识，猜测该地区为海洋，因而其反光度比其他地方较低。相对地，其他比较光亮的地方也就被称之为月陆了。此外，还有被称为湖的“月湖”；被称为湾的“月湾”；被称为沼的“月沼”。

The lunar maria /ˈmɑːriə/ (singular: mare /ˈmɑːreɪ/) are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and hence appear dark to the naked eye. The maria cover about 16 percent of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus (lake), palus (marsh), and sinus (bay). The last three are smaller than maria, but have the same nature and characteristics.