Twilight is the time period between (astronomical) dawn and sunrise, or between sunset and (astronomical) dusk. In greek we actually have two terms in order to describe this time period, while in english we describe these time periods, that occur twice each day, by using the same word; twilight.
Both of these phenomena are due to the existence of the atmosphere and during them we can distinguish both the stars and the horizon.
In the temperate zone the duration of twilight receives the highest value on 21/06 and 22/12, while the lowest is recorded on 20/03 and 23/09.
Artwork by Nikos Xatzinikos
Source; wikipedia