Simply the space between realms, made up of raw, unaligned energy of reality. It encompasses all realms, including the material world. By its very nature it is unpredictable - capable of creation or complete destruction. A power avoided by all, even the gods.
The refining of chaos through ones arcane prowess is what is commonly referred to as ‘magic’, and allows mortals to create and shape spells.
The first lesson all wizards learn - The invocation and channelling of unrefined chaos is simply not possible by mortals. It has never been achieved, and if it were to be it would simply unmake all of creation.
Some beings are seemingly born with innate magical power, and some of these appear to exhibit magics that are less refined than studied magic at will (or more often unintentionally) - known as Wild magic. These beings are exceptionally rare. While an affront to many, they are often left to their own fate, likely accidental death by their own hands.