What is the name of the stripe that separates the light side of the moon from the dark?

The stripe that separates the light side of the moon from the dark side is called the terminator. Moreover, the term terminator is used not only in relation to the Moon, but also to any other body (cosmic body), being a dividing line that separates the unlit part of the body from the illuminated one, and has a form by which it can be judged by the presence of an atmosphere on this celestial body. For the Moon, as for a spherical body, the terminator has the form of a semi-ellipse and always moves along the visible surface.