كَلَّا إِنَّهَا كَلِمَةٌ هُوَ قَائِلُهَا وَمِن وَرَائِهِم بَرْزَخٌ إِلَىٰ يَوْمِ يُبْعَثُونَ
Never! It is just a word he has to utter. And in front of them there is a barrier till the day when they will be resurrected. (23:100)
The word بَرْزَخٌ (Barzakh) means "barrier" and implies anything which intervenes between two conditions or two objects. For this reason it also means the intervening period between death and the Day of Judgment. The meaning of this verse is that the wish of the infidel who has died to be returned to this world is fruitless, because he has reached the Barzakh from which no return is possible and a second life cannot be given until the Day of Resurrection, when all the dead will rise and be brought back to life.