terminator

noun

1. a person or thing that terminates.
2. Astronomy. the dividing line between the illuminated and the unilluminated part of a satellite or planet, especially the moon.

noun

1. the line dividing the illuminated and dark part of the moon or a planet

n.1770, “line of separation between the bright and dark parts of a moon or planet,” from Latin terminator, from terminare (see terminus). Meaning “one who terminates (something)” is attested from 1846.