terminator
n. countablen. the line on a planet or moon that separates the side in the light from the side in the dark. It is the moving boundary where day meets night.
n. the dividing line between the illuminated and unilluminated parts of a planetary body or satellite. In astronomy, it refers specifically to the locus of points where the sun is on the horizon.
The moon's craters are most visible along the terminator.
Astronomers often observe the lunar surface near the terminator because the long shadows reveal the height of mountains.
As the spacecraft approached the planet, the high-resolution cameras captured the sharp contrast of the terminator, where the harsh sunlight gave way to the deep shadows of the nocturnal hemisphere.
* Partly from post-classical Latin terminator (5th century), from Latin terminō; partly from terminate + -or. * (android that kills humans): After the 1984 film The Terminator.
In astronomy, it is almost always used with the definite article 'the'.