Twilight
Twilight means either dawn (in the morning) or dusk (in the evening).
In the morning, dawn is when it is not dark any more, and the light of the sun is just starting to appear, before sunrise. In the evening, it is the other way around and that phenomenon is called dusk. At sunset, the sun disappears, and darkness is settling in. Light fades away.
The following have been defined:
Civil twilight is when the sun is less than 6deg below the horizon. It ends at sunrise in the morning or follows sunset in the evening.
Nautical twilight is when the sun is between 6deg and 12deg below the horizon.
Astronomical twilight is when the sun is between 12deg and 18deg below the horizon.
For all these definitions, the horizon is at sea level.
Other websites
Definition of Twilight , US Naval Observatory
Twilight time calculator
Formulae to calculate twilight duration , by Herbert Glarner
Parts of a day