What is a line that divides the daylit side and the dark night side of the Moon called?
A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. A terminator is defined as the locus of points on a planet or moon where the line through its parent star is tangent. An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.
The lunar terminator is the division between the illuminated and dark hemispheres of the Moon. It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation means it takes longer for it to pass across the surface.
Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when the Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows, provide accurate descriptions of the terrain.