This version of Unity is unsupported.
Method group is Obsolete

TerrainData.thickness

Obsolete Terrain thickness is no longer required by the physics engine. Set appropriate continuous collision detection modes to fast moving bodies. public float thickness;

Description

The thickness of the terrain used for collision detection.

This lets the physics engine know how thick the Terrain is when used with a TerrainCollider. Any other colliders which are no less then thickness units underneath the Terrain will be considered to collide with the terrain, and will be moved above the terrain.