sumo performed before the empress or crown prince
A formal sumo event held in the presence of the empress or crown prince, typically at the Imperial Palace.
Tairan-zumō is a special sumo event held at the Imperial Palace.
天覧相撲 is sumo performed before the emperor, while 台覧相撲 is before the empress or crown prince.
台覧 (imperial viewing by the empress or crown prince) + 相撲 (sumo).