A terminator or twilight zone is a moving line that divides the daylit side and the dark nightside of a planetary body. A terminator is defined as the locus of points on a planet or moon where the line through its parent star is tangent. An observer on the terminator of such an orbiting body with an atmospherewould experience twilight due to light scattering by particles in the gaseous layer.
A seasonal animation of the terminator line at sunset over central Europe.