What is the name for the line dividing day and night on the Moon?

The line dividing day and night on the Moon is called the terminator.

It marks the boundary between the Moon’s sunlit hemisphere and its dark hemisphere. The terminator is not a permanent feature: it sweeps across the lunar surface as the Moon rotates and as different portions become illuminated during the lunar cycle.

The Moon has no substantial atmosphere, so its terminator is much sharper than Earth’s. On Earth, air scatters sunlight and creates a gradual twilight zone; on the Moon, the transition is essentially from bright sunlight to deep darkness. The low Sun near the terminator also casts long shadows, making crater rims, mountains and small surface features especially easy to see through a telescope.

“Terminator” can also describe the day-night boundary on planets and other moons. It should not be confused with the lunar limb, which is the apparent outer edge of the Moon, or with a meridian, an imaginary line used to measure longitude.