Time necessary for thermal bleaching to half of the absorbance of the coloured form of a photochromic compound at a specific wavelength during one cycle.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 349 (https://doi.org/10.1351/pac200779030293)