From AMS Glossary
The twilight stage during which the sun's unrefracted center is at elevation angles -6° > h0 > -12°.
During a clear evening's nautical twilight, horizontal illuminance decreases from ∼3.5–2 lux to ∼0.008 lux. At nautical twilight's bright upper limit, the brightest stars are visible and the (ocean) horizon is distinct. At its dark lower limit, the horizon is generally invisible.