Light propagation volumes (LPV)
A 3D grid that indirect light is injected into and then propagated cell by cell, giving dynamic diffuse global illumination with no precomputation.
The method was developed by Kaplanyan for CryEngine 3 (2009) and extended with cascades by Kaplanyan and Dachsbacher in 2010. Indirect light — often taken from a reflective shadow map — is first injected into a lattice where each cell stores directional radiance as low-order spherical harmonics. The light is then propagated iteratively to neighbouring cells until it has filled the volume.
The appeal is that everything is fully dynamic: nothing is baked, so movable lights and geometry just work. The cost is a low-frequency result and light leaking through thin walls, because the lattice is coarse. CryEngine and early Unreal versions used it before voxel- and tracing-based GI took over.