terminator

[tur-muh-ney-ter] /ˈtɜr məˌneɪ tər/
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.
Origin
1760-70; < Late Latin terminātor, equivalent to terminā(re) to terminate + -tor -tor
Word Origin and History for terminators

terminator

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.