Critical temperature is the temperature above which a substance cannot exist in its liquid state, regardless of the pressure applied. For example, water cannot remain liquid above its critical temperature of 374°C, even if the pressure is increased. This concept is crucial in understanding phase transitions in materials. In the context of superconductivity, critical temperature refers to the temperature below which a material exhibits zero electrical resistance. This phenomenon is essential for the development of technologies like MRI machines and maglev trains, which rely on superconducting materials to function efficiently.