Diamagnetism
Definition
Weak repulsion from magnets — even water and frogs are slightly diamagnetic, and strong enough fields can levitate them.
Technical definition
Induced orbital currents oppose the applied field (Lenz's law at atomic scale); χ < 0 and tiny (~−10⁻⁵). Present in all materials but usually swamped by stronger effects.
Example
In 2000 Andre Geim's lab levitated a live frog in a ~16 T magnet — the water in its body is diamagnetic. (He later won a Nobel for graphene — and an Ig Nobel for the frog.)
Common confusion
Diamagnetism is not a separate 'anti-magnetic' force — it is the universal weak response of electron orbits, present even inside magnets but overwhelmed by stronger effects.