Diamagnetism

A form of magnetism in which the relative permeability is less than or equal to 1. 
Diamagnetic materials have a magnetic susceptibility that is less than or equal to 0, because this susceptibility is defined as χv = μv − 1.

Magnetic fields repel diamagnetic materials. They form induced magnetic fields in the direction opposite to that of the applied magnetic field.

For more information go to the Wikipedia page on diamagnetic materials.