Superconductor
Definition
A material that conducts electricity with exactly zero resistance when cooled below a critical temperature.
Technical definition
A material exhibiting zero DC electrical resistance and expulsion of magnetic fields (Meissner effect) below its critical temperature T_c. Explained by BCS theory (Cooper pairs) for conventional superconductors. Enables MRI magnets, maglev, and particle accelerators.
Example
MRI machines use superconducting magnets cooled with liquid helium — zero resistance lets them sustain the huge currents needed for tesla-strength fields.
Superconductors need extreme cold (or high pressure for new hydrides) — there is no room-temperature superconductor yet despite periodic claims. And they still have critical current and field limits; ‘zero resistance’ has conditions.