Flame Imperishable

Also known as the Secret Fire, the Flame Imperishable is the aspect of Eru Ilúvatar which is his Power of Creation. This power was with Ilúvatar at the very beginning, so when Melkor sought for it alone, it was in vain, for he could never find it because it was solely with Eru himself. Later, Eru set an aspect of it at the Heart of Arda for all of Eä.
To give life to mankind in Eä, Eru gave it to each Fëa, or soul, so that each hröa, or body, could live. It was that Gift which made all sentient beings capable of independent thought and will.

Manifestation

Fëa and Hröa, fëar and hröar in plural forms, are Quenya words for soul, or "spirit," and body.
The Children of Ilúvatar are described as existing in two parts: they have a spirit or soul, fëa which comes from the Secret Fire of Ilúvatar, and a body, hröa, which is made out of Arda. According to the Elves, the fëa is powerless without the hröa, and likewise the latter would die without the former.
The Children of Ilúvatar were not the only ones in possession of fëa, the soul was also granted to the Ents and animals, such as Huan and the Great Eagles. When Ilúvatar adopted Aulë's children, the Dwarves, as his own, he granted them, too, fëa of their own. However it is unknown what happens to the fëa of Ents, animals, and Dwarves when they die.
Ainur are, in contrast, spiritual beings that create their own physical bodies to stay in Middle-earth, and are more than capable of changing their hröa to any form as they please. However, their hröa are susceptible to harm and injury, no matter how powerful, even to the point of being completely destroyed. While an Ainu may eventually reform their hröa, their power becomes severely weakened, and such loss of power may rob them of the ability to shapeshift, and even to take any physical form at all, as seen with Sauron.

Related Deity/Higher Power

Eru Ilúvatar