The ability of certain ultra cold substances to conduct electricity without resistance is called super conductivity. This superconductivity state is a state in which a material has virtually zero electrical resistance. Substances having this property are called super conductors.
The super conducting transition temperature ‘Tc’ of a material is defined as a critical temperature at which the resistivity of the material is suddenly changed to zero. Thus at that temperature a material is changed from normal material to superconductor.
At the extremely low temperatures, vibration of the nuclei of certain atoms slow down so much and they synchronise with the passing waves of electrons in a flow of electric current. When this happens, resistance to electric current disappears.