A superconductor is a material in a state of superconductivity, characterized by dissipationless steady bulk charge transport in the appropriate regime and the equilibrium magnetic screening of the Meissner effect. The state exists only within limits on temperature, magnetic field and electric current.
A zero measured electrical resistance is an important transport signature, but magnetic measurements distinguish superconductivity from the idealized perfect conductor. Type-I superconductors and type-II superconductors differ in their equilibrium response to increasing magnetic field; a type-II material can remain superconducting while containing Abrikosov vortices. Superconducting magnets exploit persistent electric currents, while superconducting circuits exploit the Josephson effect. A material's useful operating range also depends on processing, geometry and flux pinning, not solely on its transition temperature.
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