Level streaming
Loading and unloading parts of the game world at runtime so a large world fits in memory without loading screens.
The world is split into chunks — sublevels, cells or a grid — and only those near the camera stay resident. A background thread loads what the player is approaching and unloads what falls behind, driven by distance, volumes the player crosses, or an explicit script. Done well it produces a seamless open world with no loading screen between areas, as in GTA and the Elder Scrolls games.
The hard part is hiding latency: stream far enough ahead, or the player outruns the loader and sees pop-in. Memory must be budgeted for several chunks overlapping at borders, and game state kept consistent across a seam. It is almost always paired with LOD and occlusion culling to hold down what actually gets rendered.