Tidally locked planets (Earth at Twilight)
No sudden, sharp boundary marks the passage of day into night. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience on Earth as twilight. For a tidally locked planet there is no day or night, only freezing darkness on one side and burning constant sunlight on the other. The terminator zone is bathed in constant twilight and would likely be the only place on the planet to be potentially hospitable.