Chaos is an innate power that exists within mankind in the Yu-Gi-Oh! ZEXAL anime. It is said to be the power of desire created for oneself and a force that drives primitive life to keep on living, to protect others, as well as the heart's ability to keep hoping. As such, the power of Chaos is typically found in humans and Barians. Furthermore, its existence is what led to the creation of the Barian World as any soul with such power was banished to that place.
According to Eliphas, Chaos also gives way to create negativity such as hatred and malice and can create suffering to those that misuse it. It is known to corrupt the denizens of the Astral World with darkness just as Astral did following the doubt he suffered after Yuma Tsukumo's unintended betrayal caused by Vector  causing Astral's body to start to degenerate and threatening his life. In contrast to this, Chaos can also be used to eliminate the adverse effects it causes; when Yuma's deep friendship with Astral and determined willpower was strong enough to purge the darkness within the latter's heart; which immediately causes Dark ZEXAL to reverse.
Chaos is the source of life itself, and a living thing that doesn't have chaos will wither and die. However, when something that is infused with the power of chaos touches a living thing that is not, the former's chaos will spread into the latter's body and mind, restoring them to full health. The power of chaos is never the same for two people. For example, Yuma's chaos drives his "kattobingu" spirit, while Vector's chaos turned him into a maniac.
Xyz Monsters can become infused with chaos in a process known as Chaos Xyz Evolution, which is typically used by Barians; the result is a powerful "C" monster. "C" monsters are divided into two groups: "Number C" monsters, which are upgraded versions of "Number" monsters, and "CXyz" monsters, which are upgraded versions of non-"Number" Xyz Monsters. In rare cases, "C" monsters can be upgraded further into even more powerful "iC" (Chaos Imaginary) monsters. However, only one "iC" monster is known to exist: "Number iC1000: Numeronius Numeronia".