Level of detail
Also said as LOD, Pop-in, Nanite
Using a simpler version of a model the further away it is. Done well, you never notice. Done badly, you see things change shape as you approach, which is called pop-in.
A far tree that is ten pixels wide does not need 50,000 triangles. Level of detail lets a game draw a city at the cost of a street. Someone has to make those lower versions, by hand or by tool.
Say it
“Add three LODs to every tree: full detail to 20 m, a quarter of the triangles to 60 m, then a flat billboard. Fade between them over 2 m so they do not pop.”
Where you’ve seen it
Fortnite moved to Unreal Engine 5.1's Nanite in Chapter 4, in which detail is chosen by the engine as small clusters of triangles, rather than as whole hand-made versions of each model.
Not to be confused with culling
Culling decides whether to draw something at all. Level of detail decides how much of it to draw once you have.