noun
imperial investiture ceremony
A formal ceremony in which the Emperor of Japan personally appoints a high-ranking official, such as the Prime Minister or Chief Justice. Used in official and news contexts.
新首相の親任式が皇居で行われた。
The investiture ceremony for the new Prime Minister was held at the Imperial Palace.
認証式 is a certification ceremony for officials appointed by the Emperor based on cabinet advice, whereas 親任式 is the investiture ceremony for officials directly appointed by the Emperor.
Compound of 親任 (imperial appointment) and 式 (ceremony). The term is specific to the Japanese constitutional system.