noun, noun or participle which takes 'suru', intransitive verb
imperial rites; rites conducted by the emperor
Refers to Shinto or state ceremonies personally performed by the emperor. Formal and historical in tone; not an everyday word.
天皇陛下が親祭を行われた。
His Majesty the Emperor performed the imperial rites.
祭祀 is a broader term for rituals and ceremonies, not limited to those conducted by the emperor.
Compound of 親 (しん, 'in person') and 祭 (さい, 'ritual, festival'). The exact historical derivation is uncertain, but the word is conventionally associated with imperial Shinto rites.