There are three different definitions of twilight:
Civil twilight – when the Sun’s centre is 6° below the horizon, is roughly equivalent to Lighting-up Time. In the UK, it is between 30 and 60 minutes after sunset. The brightest stars are visible and at sea the horizon is clearly defined.
Nautical twilight – when the Sun’s centre is 12° below the horizon, is to all intents and purposes the time when it is dark. For nautical purposes it is that time when the horizon ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
Astronomical twilight – when the Sun’s centre is 18° below the horizon, is when it is truly dark and no remnant of the Sun’s afterglow can be seen. It is possible to see the Zodiacal light which comes from light from the Sun reflected by small particles between the Earth and the Sun; this can be mistaken for the Sun’s afterglow.
From the Nationial Maritime Museum