An Etymological Dictionary of Astronomy and Astrophysics

The diffused light from the sky when the Sun is below the → horizon, either from daybreak to → sunrise or, more commonly, from → sunset to → dusk or nightfall.

There are three types of twilight: → astronomical twilight, → nautical twilight, and → civil twilight. They are divided on the basis of the → solar depression angle.

Etymology (EN): M.E., cognate with Du. tweelicht, Ger. zwielicht,
from twi- a combining form meaning “two, twice,” but it appears to refer to “half” light, rather than the fact that twilight occurs twice a day + → light.

Etymology (PE): Nimtâb, from nim, nimé, → half,

• tâb “light,” present stem of
  tâbidan “to shine,” → luminosity.