The Girdle of Arda was the equator of Arda, and generally corresponds to the Earth's equator.
The Girdle of Arda was shown in Tolkien's maps of Arda in the Elder Days and Second Age. It ran from the far western part of Aman through the Valar city of Valmar, across the middle of the Great Sea, the Hither Lands, the Eastern Sea, the Dark Land, and finally the mountain of Kalormë in the Burnt Land of the Sun or the Uttermost East, beyond which lay the Gates of Morning.