constituent country

Short definition from Wiktionary

Noun

1. 1
   Any country that is a subdivision of a larger sovereign state.
2. 2
   Any country that is a subdivision of a larger sovereign state: An administrative division of the United Kingdom: one of England, Scotland, Wales and Northern Ireland.
3. 3
   Any country that is a subdivision of a larger sovereign state: A land of the Kingdom of the Netherlands, of which there are four: the Netherlands proper (comprising its twelve European provinces and the three BES Islands of the Caribbean Netherlands: Bonaire, Sint Eustatius, and Saba) and the three Caribbean island countries of Aruba, Curaçao, and Sint Maarten.
4. 4
   A sovereign country as a member of a larger organization.