A formal term for an organization established by treaty or agreement between nations, such as the UN or WHO.
The WHO is one of the international organizations.
国際機関 is a near-synonym also meaning 'international organization'. 国際機構 is slightly more formal and often used in official names, while 国際機関 is more common in everyday news.
Compound of 国際 (international) and 機構 (organization, mechanism).