A material that lets electricity pass through it with no resistance at all, so no energy is lost as heat — usually only when cooled to a very low temperature. This is the countable noun for the substance itself: a superconductor, two superconductors. Certain metals become superconductors near absolute zero; a family of ceramic materials found in the 1980s works at higher temperatures, though still far colder than a freezer, and those are called high-temperature superconductors — 'high' only by comparison. What makes them useful in practice is that you can push a huge current through a superconducting coil and get a very strong magnet without it melting, which is exactly what an MRI scanner and a particle accelerator need. You will also meet the word in news stories, because a superconductor that worked at ordinary room temperature would be a genuinely world-changing discovery, and every few years someone announces one.