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member state

n. A sovereign state which is a member of some kind of confederation of other such states.

Wikipedia
Member state

A member state is a state that is a member of an international organization or of a federation or confederation.

The World Trade Organization (WTO) and the International Monetary Fund (IMF) include some members that are not sovereign states; thus, neither organization ever speaks of "members states"; the WTO has simply "members" WTO members, whereas the IMF calls its members "member countries".