Superconductors are materials that, at very low temperatures, conduct
electricity with no resistance.
This means that, unlike the more familiar conductors such as copper or steel,
a superconductor can carry a current indefinitely without losing any energy in
the form of heat, light, or sound.
Another important property is that no magnetic field can exist within a
superconductor. This property has been exploited in the creation of maglev (for
magnetic levitation) high-speed trains. This property is also used,
paradoxically, in the creation of very strong magnets used in research and in
medical applications such as magnetic resonance imaging (MRI).
Source: GrenFacts, based on Superconductor week
To read about this term in context:
GreenFacts Summary on Static Fields: