Chunk loading
Streaming a large world in fixed-size blocks, loading those near the player and unloading distant ones, so memory and cost stay bounded regardless of world size.
A world too big to hold in memory at once is split into cells — chunks. The engine keeps a ring of chunks around the player loaded and discards those that fall outside it. Because the number of loaded chunks stays roughly constant, both memory and generation cost become independent of how large the world "really" is. This is the trick behind seemingly endless procedural worlds — Minecraft's 16×16 chunks are the canonical example.
The difficulty lives in the seams and the hitches. Two chunks must line up at their shared border, or seams show in terrain and lighting. And generating a chunk on the main thread as the player runs into it freezes the frame; the fix is to generate on background threads and stay a few chunks ahead of the direction of travel.