The collateral adjective of "twilight" is crepuscular (for daylight it is diurnal and for night, nocturnal).
The term is most frequently encountered when applied to certain species of insects and mammals that are most active during that time.
In the Arctic and Antarctic regions, twilight (if at all) can last for several hours.
Most casual observers would consider the entire sky already fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse items such as nebulae and galaxies can only be properly observed beyond the limit of astronomical twilight.
Theoretically, the dimmest stars ever visible to the naked eye-those of the sixth magnitude-will appear in the evening once the sun falls more than 18° below the horizon (i.e.
At this time, sailors can take reliable star sights of well-known stars, using a visible horizon for reference.
The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult if not impossible to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk).