So, solar terminator or twilight zone is acutally an imaginary line that separates the illuminated day side of planetary body from dark night side of a body.
I will not go into details, as always, because I want it to be suitable and understandable for all ages. Links below are for more information, if anyone is interested.
|The grey line|
Looks awesome from space, I know.
NOAA - Terminator article on NOAA
Terminator - And on Wikipedia.