The period of time either just before sunrise
or just after sunset when the sky is lighted but
the Sun is below the horizon. In Washington,
D.C., Civil Twilight usually lasts
approximately 30 minutes.
Twilight is the best time to look for planets, because the planets are the
first objects (other than the Moon) to appear in the sky as the light of
day fades to darkness (or the last objects to disappear as night turns
into day).