Zero resistance means that a material does not dissipate electrical energy through ordinary resistive heating when carrying current in its superconducting state. A conventional conductor, including copper, still has some resistance and produces heat. A superconductor can support a persistent current without the same resistive loss as long as the superconducting conditions are maintained.
Researchers often test this with an electrical transport measurement, watching resistance as a sample is cooled. A sharp drop is a useful clue, but instruments have limits: a reading of zero can mean that the resistance is below the detection threshold, rather than that the experiment has established an exactly zero value with unlimited precision.
The shape of the transition matters. A reported onset temperature is where resistance first begins to fall; the zero-resistance temperature may be lower. Contact problems, sample inhomogeneity and pressure-cell measurement difficulties can make results harder to interpret. Magnetic measurements provide complementary evidence for superconductivity.
When evaluating a claim, ask whether zero resistance was actually reached or only inferred from an onset, how the contacts and background signals were checked, and whether the result repeats across samples. Also check the magnetic field and applied current, since sufficiently high values can end the superconducting state.