A transition in which the molar Gibbs energy (or chemical potential) and its (n - 1)th-order derivatives are continuous, whereas the nth-order derivatives with respect to temperature and pressure are discontinuous at the transition point.
See: first-order phase transition, second-order transition
Source:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 587 (https://doi.org/10.1351/pac199466030577)