A member state is a state that is a member of an international organization, federation, or confederation.
Since the World Trade Organization (WTO) and the International Monetary Fund (IMF) include some members that are not sovereign states, neither organization ever speaks of "member states". The WTO has simply "members" (see WTO members), and the IMF refers to its members as "member countries".
The oldest global member state based organization is the International Telecommunication Union, which joined the United Nations System as a specialized agency after the creation of the UN.
In history: