induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
Chassis ground means the ground is connected to outer body of appliances so that all charge can be neutralized and safe. earth ground means the top hole of mains socket which is connected to a metal piece via copper wire covered in sand inside the ground it is done to discharge extra charges.