1.
member state; member nation
A country that belongs to an international organization, treaty, or alliance. Frequently used in the context of the United Nations, EU, and other international bodies.
EU加盟国は二十七か国ある。
There are 27 EU member states.
新たな加盟国を迎えた。
A new member state was welcomed.
国連加盟国の代表が総会に集まった。
Representatives from UN member states gathered at the general assembly.
Composed of 加盟 (joining, affiliation) + 国 (country). Common collocations: 加盟国を増やす (to increase member states), 加盟国の義務 (obligations of member states). Often preceded by the organization name: 国連加盟国 (UN member state), EU加盟国 (EU member state), NATO加盟国 (NATO member state). Compare with 加盟する (to join/become a member).