noun
terminator; twilight zone
Astronomical term for the dividing line between the illuminated and dark portions of a celestial body. Also called the twilight zone.
月の明暗境界線は、クレーターの観察に適している。
The lunar terminator is suitable for observing craters.
Compound of 明暗 (meian, 'light and darkness') + 境界線 (kyōkaisen, 'boundary line').