If the electrical resistance of a typical substance drops to zero, the…

Electrical resistance is the opposition a material offers to the flow of current. In ordinary conductors, resistance falls as temperature drops but never becomes exactly zero. A superconductor is a specific class of material that, below its characteristic critical temperature, offers exactly zero electrical resistance to a steady current, so current can flow indefinitely with no energy loss (accompanied by the Meissner effect of expelling an external magnetic field).

Here, the substance's resistance is stated to drop all the way to zero. An exact zero-resistance state is the defining property of a superconductor, so the substance described is a superconductor.

• A semiconductor's resistivity lies between that of conductors and insulators, and it typically decreases as temperature rises because more charge carriers get thermally excited across the band gap; it never reaches an exact zero this way.

• An ordinary conductor (metal) has low resistivity that decreases somewhat as temperature falls, but it retains a small residual resistance from lattice vibrations and impurities even at very low temperatures.

• An insulator has extremely high resistivity because very few charge carriers are available to move under an applied field, so it strongly opposes current rather than offering none.

This matches standard physics references (e.g. NIST and introductory college-physics texts), which define a superconductor precisely as a material with zero electrical resistivity below its critical temperature.