What is the name for a material that has zero electrical resistance?

A material that has zero electrical resistance is called a superconductor.

A superconductor enters a special state when cooled below its critical temperature. In that state, its electrical resistance drops abruptly to zero, allowing a current to continue without the ordinary energy loss associated with resistance. This is different from a normal conductor, such as copper, whose resistance can decrease as it cools but does not ordinarily vanish.

Superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes while studying mercury cooled with liquid helium. He observed the resistance disappear at about 4.2 kelvin. Later research showed that superconductivity also involves the Meissner effect: a superconductor expels magnetic fields from its interior under suitable conditions.

A common misunderstanding is that every superconductor works at room temperature. Each material has limits involving temperature, magnetic field and current; exceeding them destroys the superconducting state. Applications include MRI magnets, particle accelerators, sensitive SQUID detectors and magnetic-levitation demonstrations. “Superconductor” names the material, while “superconductivity” names the phenomenon or state it exhibits.