Diamagnetism is the property of an object or material which causes it to create a magnetic field in opposition to an externally applied magnetic field.
Diamagnetism is believed to be due to quantum mechanics (and is understood in terms of Landau levels) and occurs because the external field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, the field of these electrons will oppose the magnetic field changes provided by the applied field.
In most materials diamagnetism is a weak effect, but in a superconductor a strong quantum effect repels the magnetic field entirely, apart from a thin layer at the surface.
Diamagnets are materials with a magnetic permeability less than Failed to parse (Missing texvc executable; please see math/README to configure.): \mu_0
In 1778, Sebald Justinus Brugmans was the first individual to observe that bismuth and antimony were repelled by magnetic fields. However, the term diamagnetism was coined by Michael Faraday in September 1845, when he realized that every material responded (in either a diamagnetic or paramagnetic way) to an applied magnetic field.