Image-based lighting (IBL)
Lighting a scene with a picture — often a panorama of a real place — instead of a handful of point lights, so objects pick up colour and reflections from their surroundings.
The idea comes from Paul Debevec's 1998 work on high-dynamic-range light probes: photograph an environment in every direction at full brightness and use it as the light source. An object rendered this way picks up the blue sky from above, the warm floor from below and mirrors the windows behind the viewer, without anyone placing the lamps by hand.
Integrating the whole environment per pixel is too costly in real time, so Unreal popularised a split in 2013 into two precomputed parts: a cubemap blurred by varying amounts for different roughness, plus a small lookup texture for the BRDF. The diffuse part is often stored as spherical harmonics. IBL is what gives PBR materials their ambient light and reflections.