Il˙vatar's power or spirit of Creation, the "Holy Spirit", by which the Ainur and Eń were created.
Also called the FlameImperishable and the SecretFire.
The Sun was probably a symbol of the SecretFire, hence called the Flame of Anor, the power of Gandalf...
Therefore Il˙vatar gave to their vision Being, and set it amid the Void, and the Secret Fire was sent to burn at the heart of the World; and it was called Eń.
From: The Silmarillion
You cannot pass,' he said. The orcs stood still, and a dead silence fell. `I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udűn. Go back to the Shadow! You cannot pass.'