A term that appears just once, in Bilbo's "Song of Eärendil" quoted above. The meaning is not completely certain, but it seems to describe the Pelóri, the Mountains of Defence that ranged along the eastern borders of Valinor.

The Mountain Wall of the Pelóri was raised by the Valar after they came to Aman in the West of the World. With their earlier dwelling on Almaren destroyed by Melkor, they sought to protect themselves against him by building a wall of mountains along their eastern shoreline that ran from the far north of Aman into the distant south. Taniquetil, the tallest mountain in Arda, stood in the middle of that range, and on its peak Manwë and Varda dwelt in the halls of Ilmarin.

When the Elves came to Aman long afterwards, they missed the sight of the starlight in the world beyond the Light of the Trees, and so the Valar created a narrow opening in the Mountain Wall, known as the Calacirya or the Pass of Light, and in that Pass the Elves built their city of Tirion.

So things stood for three ages, until Melkor destroyed the Two Trees and fled from Valinor with the Silmarils. At that time the Valar redoubled their defences, raising the already great Mountain Wall to vast heights, and making its outer face unclimbably smooth and steep.