A transition at the critical temperature, T c, below which the resistance of electrical conductors becomes zero and magnetic flux is excluded. Examples: The transitions of Nb3Sn
to superconductors (15 K < T c < 20 K) and YBa2Cu3O7-x
(x ≈ 0.2) at 90 K.
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 592 (https://doi.org/10.1351/pac199466030577)