absorption which takes place with the transition from the lowest triplet state of the molecule to higher triplet states (T → T n) thus leading to the triplet-triplet absorption spectrum.
Source:
PAC, 1984, 56, 231. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy' on page 234 (https://doi.org/10.1351/pac198456020231)