Heightfield collider
A collision shape for terrain that stores only one height per grid point and leaves the triangles implied.
A 1025 × 1025 terrain turns into two million triangles as a regular mesh collider, with the memory and search tree to match. A heightfield stores just the heights instead, often as 16-bit integers. Which cell a point sits above falls straight out of its X and Z coordinates, so the physics engine finds the two relevant triangles without searching. PhysX, Bullet, Jolt and Godot all ship a dedicated shape for it.
The limitation comes with the design: every point has exactly one height, so overhangs, arches and caves can't be expressed. They get built as separate mesh colliders on top, and the seam between the two is where players traditionally fall through the world.