The temperature (more precisely, narrow temperature range) above which the solubility of a surfactant rises sharply. At this temperature the solubility of the surfactant becomes equal to the critical micelle concentration. It is best determined by locating the abrupt change in slope of a graph of the logarithm of the solubility against t or 1/T.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 613 (https://doi.org/10.1351/pac197231040577)