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.
Update: Pokemon Weaknesses Fire is weak against water, ground and rock Grass is weak against fire, poison and flying Steel is weak against ground, fire and fighting Ice is weak against steel, rock, fire and fighting Ground is weak against water, ice and grass Fighting is weak against flying and psychic Ghost is weak...