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.

A practical example of diamagnetism:

Water is weakly diamagnetic, about forty times less than, for example, pyrolytic carbon. This is enough for light objects - containing a lot of water - to float if they are in a strong magnetic field.

This frog started floating for example, using a 16 tesla electromagnet at the High Magnetic Field Laboratory at Radboud University Nijmegen in the Netherlands.

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