Wiktionary
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".