A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body. A terminator is defined as the locus of points on a Moon or planet where the line through a Sun is tangent.
The Earth's terminator 
On Earth, the terminator is a circular line with a diameter that is approximately that of the Earth. The terminator passes through any point on the Earth's surface twice a day, once at sunrise and once at sunset, apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night. The line separates the portions of the Earth experiencing daylight from the portion of the planet experiencing darkness. While one half of the Earth is illuminated at any point in time (with exceptions during eclipses), the location of the terminator line varies by time of day due to the rotation of the Earth on its axis as well as the revolution of the Earth around the Sun. The terminator line also varies by time of year: on Earth the angle of the line is almost parallel to lines of longitude during the equinoxes, and at its maximum angle of approximately 23.5 degrees to the equator during the solstices.
At the equator, under flat conditions (no obstructions such as mountains; or at a height above any such obstructions), the terminator line moves at approximately 1600 kilometers per hour (1000 miles per hour). This speed can appear to be increased when near obstructions—such as the height of a mountain, for example—as the shadow of the obstruction will be cast over the ground in advance of the terminator line along a flat landscape. The speed of the terminator line decreases as one approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).
Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic transports are the only aircraft able to overtake the maximum speed of the terminator. However, slower vehicles can overtake the terminator at higher latitudes and it is possible to walk faster than the terminator at the poles, near to the equinoxes. The visual effect is that of seeing the Sun rise in the west.
Amateur radio operators take advantage of conditions at the terminator (so-called "grey line" conditions) to perform long distance communications. Under good conditions, radio waves can travel along the terminator to antipodal points. This is primarily because the D layer, which absorbs high frequency signals, disappears rapidly on the dark side of the terminator line. This process is known as skywave propagation.
The transition from light to dark takes place on two fronts in this image of Mimas.
The orientation of the terminator between the northern and southern hemispheres depends on the season. On the spring and fall equinoxes (around March and September 21), there is no tilt of the Earth with respect to the Sun so the terminator line is parallel with the axis of the Earth and with the lines of longitude.
Scientific significance 
Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere can create a fuzzier terminator. As the particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the Sun has set.
Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse, therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit. This prolongs the operational life of a LEO satellite, as onboard battery life is prolonged. It also enables specific experiments that require the minimum influence of the Sun, as the designers can opt to install the relevant sensors on the dark side of the satellite.
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- Current terminator
- aa.usno.navy.mil - Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)