In the Dungeons & Dragons campaign setting Forgotten Realms, the Weave is the fundamental force of both arcane magic and divine magic, from which spellcasters draw their strength. It permeates the world, comparable to aether, the mystical substance.
The Weave acts as both a barrier and a gate between raw magic and the world. With the Weave in place, mortals can access magic and are protected from the harmful effects of contacting raw magic while still able to tap its refined energy. With the Weave gone, raw magic quickly dissipates from the world, but only after disrupting existing magic and lashing out at nearby magical conductors. Both times that the Weave has been destroyed (with the death of Mystryl and the first Mystra), widespread destruction resulted, and the Weave's return was only partially able to correct this, with some portions of the Weave left absent or damaged.
The Weave is present in everything in Faerûn, whether living, dead, undead, inanimate, solid, liquid, or gas. It permeates the soil, suffuses the deeps of the ocean, and stretches to the limits of the air in the sky and beyond. The Weave is like a great base fabric upon which the substance of the world is embroidered, a web upon which mortals walk like spiders, a great ocean upon which all objects float. At the same time, it is an aspect of Mystra, and it is by her will that the Weave reaches where it does.
Spellcasters, knowingly or not, cause spells to work by drawing power from the Weave, adjusting the balance of energy so that the power of the Weave comes forth and is shaped by their actions and will. When the Weave is damaged by reckless spellcasting or magical disaster, it can be torn, shredded, or destroyed, leaving dead magic and wild magic areas.
Also it is said that the Weave is Mystra herself. If a spellcaster has been abusive of magic, trying to inflict great harm to magic itself (such as by intentionally creating areas of wild or dead magic), or researching spells of mass destruction, Mystra will negate the spellcaster's ability to manipulate the Weave, leaving them unable to cast spells, use spell components, enchanted items or activate spell-like or supernatural abilities.
It is tended by the goddess Mystra. In relatively recent times, the goddess Shar created a rival Shadow Weave in response to Selûne's creation of Mystra and the birth of the Weave. Shar, being the goddess of secrets, has mostly kept the secret of the Shadow Weave to herself. Over the millennia some mortals, mainly her servants, have been allowed to discover the Shadow Weave or have stumbled across it in their researches. In the Return of the Archwizards series, however, the Shadow Weave was created when Karsus attained godhood, causing the Weave to split in two.