A domain wall is a theoretical 2-dimensional singularity. The term is widely used in theoretical physics, especially in string theory. A domain wall is meant to represent an object of codimension one embedded into space (a defect in space localized in one spatial dimension). For example, D8-branes are domain walls in type II string theory. In M-theory, the existence of Horava-Witten domain walls, "ends of the world" that carry a E8 gauge theory, is important for various relations between superstring theory and M-theory.
Domain walls are topological solitons which occur whenever a discrete symmetry is spontaneously broken.
See also soliton (topological), topological defect.
Domain walls have also been referenced in science fiction novels. Stephen Baxter's Xeelee series suggests domain walls being used to power spacecraft, through manipulation of gravity.