noun
legation
A diplomatic mission headed by a minister, ranking below an embassy. Historically common; now largely replaced by embassies.
戦前、日本は多くの国に公使館を置いていた。
Before the war, Japan maintained legations in many countries.
大使館 is an embassy, a higher-ranking diplomatic mission headed by an ambassador. 公使館 is a legation, now mostly historical.
Compound of 公使 (minister) and 館 (building, hall). The term reflects the historical diplomatic rank of minister.