means A material that, when cooled below a critical temperature, conducts electricity with exactly zero resistance, so a current once started flows indefinitely without losing energy.

from A modern compound from the Latin pieces 'super-' (above, beyond) and 'conducere' (to lead together, from 'con-' plus 'ducere,' to lead). 'Conductor' was already doing service in physics for anything that carries current; the 'super-' was bolted on after 1911, when the Dutch physicist Heike Kamerlingh Onnes chilled mercury near absolute zero and watched its resistance vanish entirely — a phenomenon he named 'superconductivity.' So the word is barely a century old, built from very old roots to describe something nature had been hiding in the cold the whole time.