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.
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).