The ability of certain materials to carry electricity with no resistance at all — usually only when they are made extremely cold. Normally, when electricity flows through a wire, the wire fights it a little and some of the energy is lost as heat; that is why a charger gets warm. In superconductivity that loss drops to zero: start a current going in a closed superconducting loop and it will keep going round and round without fading. Another famous effect comes with it — the material pushes magnetic fields out, so a magnet placed above it floats in mid-air. This is a physics and engineering term, uncountable, and it names the property or the phenomenon, not the material. Real uses today include the magnets inside hospital MRI scanners and inside particle accelerators. The great open goal of the field is a material that does this at room temperature, which would change how the world moves electricity; claims of it appear in the news every few years and have so far not held up.