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:
For all these definitions, the horizon is at sea level.