light cone

The solid cone in space-time swept out by every point a single source can ever affect, once the wave equation's speed limit is extended to three spatial dimensions.

For the 3D wave equation, a source at $(x_0,0)$ can influence only the solid forward cone $\{(x,t):|x-x_0|\le ct,\,t\ge0\}$ -- the direct 3D generalization of the 1D range-of-influence wedge, named for the identical cone that bounds which events light from a single flash can ever reach in special relativity.