Nautical Twilight
When it's dark, but also not dark.
Nautical twilight is a specific time period inside the period we call twilight.
Twilight
Twilight is the period between sunset and dusk, and between dawn and sunrise. The period when the centre of the sun touches the horizon until the moment it gets completely dark, after that is when the night starts.
Nautical twilight
Nautical twilight is after the geometric centre of the sun is 6° below the horizon until the centre of the sun is 12° below the horizon. During this periode the horizon is clear to see, and the remaining light is not too bright to not see the stars and planets. This is the optimum time to do Celestial Navigation with a Sextant
To the human eye the world in the shaded parts will become grey scale, while the direction of the first or last sunlight might still be in colour.
Other twilights
Civil twilight
Civil twilight is from when the centre of the sun goes below the horizon until the centre of the sun is 6° below the horizon.
It's the moments before sunrise when the world starts to come out of grey-scale and into full colour vision.
Civil dawn and dusk is often used by legislators to define when vehicles must show illumination, but usually the time before or after sunset is given in minutes rather than degrees.
Astronomical twilight
Astronomical twilight is from when the centre of the sun is 12° below the horizon until 18°, after that it is night.
During astronomical twilight, the sky has a faint glow. This may not be visible anymore in urban areas with a lot of light pollution. After, and before, astronomical twilight, the sky is dark enough to see the faintest stars.
Why in degrees
The numbers are given in degrees, and not in minutes, because the time it takes for the sun to go that many degrees below the horizon differ per season and location on earth.
Around the Equator (in between the Tropic of Cancer and the Tropic of Capricorn) when the sun passes straight over a persons head at noon, and goes down almost vertically the time twilight takes is the shortest. Close to the Arctic Circle and the Antarctic Circle twilight can last the whole night, or not even happen at all.
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