God. Dwelling alone in the Timeless Halls, Ilúvatar created the Ainur with the Flame Imperishable of his spirit, and he revealed to each of them a part of his mind. Desiring to make their comprehension more complete, he revealed to the Ainur the three great themes of the Ainulindale. When Melkor created discord within that Music, but others of the Ainur were drawn to the Vision which came from it, Ilúvatar gave it Being, and Ea was created.
Within Ea, the Valar independently tried to fulfill the Music, but some things - the creation of Men and Elves, the destiny of Men, and the End, for example - remained known only to Ilúvatar. He intervened directly in affairs with Ea only twice: to santify Aule's creaton of the Dwarves, and to change Arda at the request of the Valar, when Numenoreans landed on Aman.
Ilúvatar is the name given him in Arda; he is also called Eru, Eru Ilúvatar, the One, and (once) God.
From The Complete Guide to Middle-Earth.