Mesh collider
A collider whose shape is an arbitrary triangle mesh instead of a primitive like a box or capsule.
When a box, sphere or capsule can't describe the shape, the physics engine can collide directly against a triangle mesh. This is the usual choice for static level geometry — terrain, walls, staircases — where accuracy is worth the cost. The engine builds a BVH over the triangles so a query doesn't have to test every one.
The price is performance and restrictions. A concave mesh collider can normally only be static; to give a moving body a mesh shape you must first split it into convex pieces with convex decomposition, because most narrow-phase algorithms assume convexity. So a falling object gets a capsule or box as an approximate hull, while the immovable world carries the expensive triangles.