A superconductor is a material that at a certain temperature has no electrical resistance. This temperature is called the critical temperature. Magnetic fields are also expelled below a superconductor's critical temperature, as explained by the Meissner effect. Standard conductors such as copper, aluminum, or gold cannot superconduct due to impurities along with other defects and will show resistance even near absolute zero. The resistance of a superconductor drops abruptly to zero when it is cooled below its critical temperature (SQUIDs, Gallop).
An electric current can flow indefinitely in a loop of superconducting material. This is because with no resistance, the current has nothing slowing it down or resisting its flow, which would normally generate heat.
