noun
flag state
Technical term in maritime and aviation law referring to the country where a vessel or aircraft is registered and whose flag it flies.
その船の旗国はパナマです。
The flag state of that ship is Panama.
旗国は、自国に登録された船舶に対して安全規制を課す責任がある。
The flag state is responsible for imposing safety regulations on vessels registered in its territory.