Texture streaming
Load only the texture resolutions that are actually visible right now instead of keeping every texture at full resolution in video memory.
An open world holds more texture data than fits in video memory. Texture streaming keeps low mip levels resident at all times and streams in the high-resolution levels only once the camera is close enough for them to matter, evicting them again when you move away.
The price is visible pop-in: a surface may linger a moment in a blurry low resolution before the sharp level arrives — most obvious right after a fast camera cut. The technique is close kin to virtual texturing and to id Software's MegaTexture, which pushed the idea to its limit.