vice-envoy; deputy delegate
Official title for a deputy or assistant envoy in diplomatic or formal missions.
彼は遣唐使の副使として唐に渡った。
He went to Tang China as the vice-envoy of the Japanese mission.
会議には大使
The deputy delegate attended the conference on behalf of the ambassador.
Compound of 副 (vice-, assistant) and 使 (envoy, messenger). The exact historical derivation is uncertain, but the word follows a common pattern for official titles.