The "twilight zone" is a line between the illuminated day side and the dark, night side of a planetary body, more correctly known as the terminator. The terminator appears on every planet as a circular line with a diameter that is approximately same of that planet. Apart from polar spots, the terminator passes through any point on the planet's surface twice a day...at sunrise and sunset...and terminates or commence new line of a planetary body.

Twilight is the time between dawn and sunrise, and the time between sunset and dusk. Sunlight is scattered in the upper atmosphere and illuminates the lower atmosphere. The surface of the planet is neither completely lit nor completely dark...it's being in twilight.

Earth terminator

The terminator is the locus of points on a planet where the line through the local Sun is tangent. The line separates the portions of the planet with daylight from the portion of the planet with darkness. While one half of that planet is illuminated at any point in time (except during eclipses), the location of the terminator line varies by time of day due to the rotation on its axis as well as the revolution of the earth around the sun. The terminator line also varies by time of planetary body and constellation of galaxy. The angle of the line is being almost parallel to lines of longitude during the equinoxes. During the solstices the angle at its maximum is approximately 23.5 degrees.

>> Dusk seen from a plane...