critical temperature is the temperature below which a material becomes superconducting, abruptly losing all electrical resistance.
Written T_c, the threshold is a fixed property of each material. Mercury, the first known 📝superconductor, superconducts below 4.2 kelvin; niobium-titanium below about 9 kelvin; niobium-tin below about 18 kelvin. For seventy-five years every known critical temperature sat within roughly 20 degrees of absolute zero, until Georg Bednorz and Alex Müller's 1986 discovery of superconducting copper-oxide ceramics opened a new class of materials whose thresholds run an order of magnitude higher — REBCO superconducts below roughly 90 kelvin, above the 77-kelvin boiling point of liquid nitrogen. That gap is the entire meaning of the "low-temperature" versus "high-temperature" division among superconductors.
Critical temperature is one of three limits bounding the superconducting state, alongside the critical current and the critical magnetic field, and the three trade against each other: the colder a superconductor runs below its T_c, the more current it carries and the stronger the field it tolerates. This is why 📝fusion magnets wound from 📝High-Temperature Superconducting (HTS) Magnets material are operated around 20 kelvin rather than at 77 — not because the higher temperature is unreachable, but because the margin bought by running cold is what lets the conductor survive fields of roughly 20 tesla.