Veil of Arda

Veil of Arda means probably all the atmosphere surrounding Arda, the special dominion of Manwë, mentioned in the Valaquenta:

In Arda his delight is in the winds and the clouds, and in all the regions of the air, from the heights to the depths, from the utmost borders of the Veil of Arda to the breezes that blow in the grass.

It is still possible that here "Veil of Arda" only refers to the outermost region beneath the Ilurambar, Vaiya, and not to the subjacent regions, Ilmen and Vista.