Cascaded shadow maps
Splits the view frustum into near-to-far slices, each rendered into its own shadow map, so a single sun casts crisp shadows up close and cheap ones far away.
A single shadow map over the whole scene wastes resolution: near objects, which fill most of the screen, get too few texels and the shadow edge turns into a staircase. CSM — also called parallel-split shadow maps, developed by Engel and Zhang among others around 2006 — gives each frustum slice its own map sized to its share of the screen.
It is the standard for directional sunlight in virtually every modern engine. The thresholds between cascades must be blended to hide the seams, and a slice chosen too tight produces peter-panning, where the shadow detaches from its owner's feet like Peter Pan's own.