night: the dark part of the day, and by extension the spiritual darkness of the present age before God's coming day
At its plainest, this word means night, the stretch of darkness between sunset and dawn. The New Testament uses it for ordinary things that happen after dark: travel, fishing, prayer, arrests, escapes, a secret visit to Jesus, the hour Judas left the supper table when, as John pointedly adds, it was night.
Night is also the natural partner of day, and the two together became a stock phrase for round-the-clock devotion or unceasing trouble. When the New Testament says someone served God or worked or wept night and day, the point is not the dark hours as such but that the activity never stopped.
From there the writers stretch the word into a picture. Because night is the time of sleep, blindness, and danger, it becomes an image of the present age of spiritual ignorance and moral darkness, set against the coming day of salvation. Paul says the night is far gone and the day is near, and that believers are children of the day, not of the night. Jesus warns that night is coming when no one can work. The image runs all the way to its end in Revelation, where the New Jerusalem has no night at all, because God himself is its light.
The same plain word carries all of this. Whether literal or figurative, it is always night that is meant, which is why one English word covers the whole range while the surrounding text tells the reader which shade is in view.