The Quran

وَالَّذينَ آمَنوا وَعَمِلُوا الصّالِحاتِ سَنُدخِلُهُم جَنّاتٍ تَجري مِن تَحتِهَا الأَنهارُ خالِدينَ فيها أَبَدًا ۖ لَهُم فيها أَزواجٌ مُطَهَّرَةٌ ۖ وَنُدخِلُهُم ظِلًّا ظَليلًا

Muhammad Asad

But those who attain to faith and do righteous deeds We shall bring into gardens through which running waters flow, therein to abide beyond the count of time; there shall they have spouses pure: and [thus] We shall bring them unto happiness abounding.¹

The primary meaning of zill is "shade", and so the expression zill zalil could be rendered as "most shading shade"- i.e., "dense shade". However, in ancient Arabic usage, the word zill denotes also "a covering" or "a shelter" and, figuratively, "protection" (Raghib); and, finally, "a state of ease, pleasure and plenty" (cf. Lane V, 1915 f.), or simply "happiness" - and in the combination of zill zalil, "abundant happiness" (Razi) - which seems to agree best with the allegorical implications of the term "paradise".