Two atoms or molecular entities both in a triplet state often interact (usually upon collision) to produce one atom or molecular entity in an excited singlet state and another in its ground singlet state. This is often, but not always, followed by delayed fluorescence.
See also: annihilation (in radiochemistry, in photochemistry), spin conservation rule
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2281 (https://doi.org/10.1351/pac199668122223)