What name is given to the Moon's dark basalt plains?

The Moon’s dark basalt plains are called lunar maria.

“Maria” is the plural of mare, the Latin word for “sea.” Early telescopic observers thought the Moon’s dark patches might be seas, but they are actually broad plains of solidified basaltic lava. The basalt erupted from the lunar interior and flooded ancient impact basins, producing surfaces that reflect less light than the brighter lunar highlands.

The maria formed mainly between about 3 and 3.5 billion years ago, when the Moon was much more volcanically active than it is today. They cover roughly 16% of the lunar surface and are concentrated on the side that faces Earth. Examples include Mare Tranquillitatis, where Apollo 11 landed, and Mare Imbrium, one of the largest impact-basin plains.

A common mistake is assuming “dark” means wet: lunar maria contain no seas of liquid water. They are also different from lunar rays, the bright streaks of ejecta radiating from some craters, and from the rugged highlands surrounding them.