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Arc is one of several known manifestations of the Light, along with Solar and Void. It is related to electricity, and can be wielded by Guardians to generate lightning bolts and other electromagnetic phenomena. It is also one of four classes of elemental damage common to all Guardian weapons.
Many of the Golden Age technologies of Humanity possesses circuitry and generators that runs on Arc Energy, including weapons. Vex are known to have a hindrance to exposure to Arc Energy, although this would be if they weren’t attempting to harness it.  Arc is even found in the weather system itself. Arc energy can be found in Dark Ether with how the Screebs and Abominations use their attacks.
List of appearances