Terminator strip

A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of any celestial real estate. A terminator is defined as the locus of points on a Moon or planet where the line through a Sun is tangent.

Frequently the phrase "Terminator Strip" in the Union refers specifically to the one on Twilight.

Twilight is a tidal locked moon around Gigaball, the fourth planet in the Yutoo System. Twilight has an unmoving terminator strip. Because of this, the daylight side experiences temperatures up to 250 degrees Celsius and the night side is at minus sixty degrees. A strong permanent wind circulation between the two sides has smoothed much of the rocky surface and created a canyon separating the two. Fresh, liquid water exists there and developed indigenous plant life. Most cities and settlements on Twilight are located on or under the terminator strip.