Eä (Q."it be!") in the closer sense refers to "Nature" or the "created Universe", Cosmos ,the world that is, though it usually only referred to the material world in contrast to Ilu, which includes the spiritual realm. Eä does not include Illuvatar himself nor the Souls and Spirits.
The possible future, "mended World" after the Dagor Dagorath is sometimes referred to as Yéva by the High Elves.