Weave

The Weave is the means by which raw magic is tapped into and used by casters. The Weave is the way in which magic presents itself to beings for their use, and it flows throughout the world, touching almost every corner or existence, with exception of dead magic zones.

Nature of the Weave
The Weave is considered many things, including Mystra's body, the source of magic, all the studies of casters, arcane and divine alike, and the many energies and forces that exist around the planes.

Casting a spell is merely telling the Weave to rearrange itself to create an effect.

Anomalies
Places such as dead magic zones or wild magic zones are places where the Weave has been damaged or is non-existent.

Wild magic zones
In wild magic zones, a spell can backfire upon its caster, target the wrong location, be dramatically increased in power, or many other things. A wild magic zone and its parameters can be detected using a Detect Magic spell. A caster can temporarily fix a wild magic zone with any dispel or permenantly fix it using a Wish spell.

Dead magic zones
Dead magic zones are places where the Weave is no longer in existence. Both it and its boundaries can be detected in the same way a wild magic zone can be detected, but from outside the zone. However, unlike wild magic zones, most casters innately know when they've stepped into a dead magic zone. A Dead magic zone can only be fixed with a Wish spell.