The Power Cosmic is a force of energy shared by primal entities and creatures described as gods. It is also called the 'Light of Creation', and is the source of power for the Heavenly Host. It springs from the Source, the singularity where all points of the multiverse meet.
It is a source of fundamental energy, allowing for primal forces to shape reality according to their strength of will and as their duties mandate. Harmony with the universe and willpower determine the effectiveness of the Power Cosmic-- primal entities often work through intermediaries to promote a harmony most akin to their individual mandate in order to strengthen themselves.
The First Gods were creatures composed entirely of the Power Cosmic, who dwelt along the interstitial space between the Source and the rest of the universe. When the Godkiller weapons were used, this space was rendered uninhabitable.
Galactus channels the Power Cosmic, as do the Celestials. Humans invoking the Power Cosmic often work a form of magic that allows them to emulate Enochian, creating buffers and gates to give the power shape and function.
Primarchs among the New Gods are capable of wielding the Power Cosmic to serve their will. At the heart of their demesnes, the primarch functionally has unlimited willpower because they are in a state of maximum harmony with their surroundings.
The act of true creation requires channeling the Power Cosmic and shaping the raw material of the Astral Plane into reality. The language of the First Gods is called Enochian and serves as a means of shaping and channeling these tremendous energies more efficiently.