The name in the Quenya tongue for the Moon, given to it by the Vanyar at the time it was created. The name refers to an object that shines with bright silvery light, but in this context is usually translated simply 'the Sheen'.

According to the tradition of the Elves, the Moon first appeared after the Darkening of Valinor and the destruction of the Two Trees that had given Light to Valinor. Telperion the White Tree put out a single silver flower as it died, and the Valar took this flower and from it they formed Isil the Moon. Set in a vessel created by Aulë, it was steered into the sky by a hunter named Tilion. Though set to follow a path through the high airs of Ilmen, Tilion followed an uncertain course above the world, and so Isil was thus sometimes called Rána by the Elves, meaning 'the Wayward'.

The name Isil is probably best known not for the Moon itself, but for forming part of Isildur, the name of Elendil's eldest son. That name meant 'devoted to the Moon', and in Middle-earth Isildur established himself in a tower named Minas Ithil ('Moon-tower') in a part of Gondor named Ithilien ('Moon-land'). These names use Ithil, the Sindarin equivalent of Quenya Isil.