Twilight
As I discussed in this post, although there is one definition of night in the FARs (Night means the time between the end of evening civil twilight and the beginning of morning civil twilight, as published by the American Air Almanac, converted to local time.) there are lots of rules that we generally consider as applying to night flight that don’t use the definition.
What may not be as well known is that there are other kinds of twilight: nautical and astronomical. And civil twilight wasn’t given a precise definition until the early 20th century. This article about the Apple Watch gives an overview of the three types of twilight and the reasons for their existence.
In order, therefore, the sequence starting at Sunset is: Sunset, Civil Twilight, Civil Dusk; Nautical Twilight, Nautical Dusk; Astronomical Twilight; Astronomical Dusk … and finally, Night proper. This is followed by Solar Midnight, which is the moment when the Sun is at its nadir on the celestial sphere from the standpoint of the observer. They all depend on the apparent distance of the sun below the horizon. 6°, 12°, and 18° respectively with dusk being the moment when the sun is exactly at these distances. The reverse order applies to sunrise.
As an aside, technically the phrase dusk to dawn is the same as the FAA definition of night.