noun
host country; chair country
Refers to the country that presides over or hosts an international conference, summit, or organization. Often used in diplomatic and political contexts.
今年のG7サミットの議長国は日本です。
The chair country for this year's G7 summit is Japan.
議長国として、会議の進行役を務める。
As the host country, it serves as the facilitator of the meeting.
主催国 is the organizing or sponsoring country, which may or may not be the same as the chair country.
Compound of 議長 (gichō, 'chairperson') and 国 (koku, 'country'). The term is a modern coinage used in international relations.