In the 1600s, the Moon was divided into two terranes, terra and maria.  The terra terrane was thought to be landmass, and the maria terrane was thought to be the Moon's ocean, although this is now known to be false.  The maria terrane is lower in elevation and younger in age than the terra terrane, and was formed by lava.  The terra terrane is higher and older, and hence more cratered.  Visually, the maria correspond to the dark regions of the Moon, and the terra to the light.