The Imperishable Fire:

The source of all life. It was with the Imperishable Fire that Ilúvatar created the Ainur. Melkor searched for it for a long time, but found it not, because Ilúvatar kept it with him. When he created Arda, Ilúvatar set the Fire at it's heart. Another name for the Imperishable Fire is The Secret Fire: "I am a servant of the Secret Fire, wielder of the flame of Anor." (LOTR.348)