Plain English Meaning
A worldwide body that sets the common rules countries follow so that flying between them works the same way everywhere.
Full Definition
A specialized agency of the United Nations that develops international standards and recommended practices for civil aviation, including rules for airspace, aircraft operations, air traffic services, communication procedures, and airport design. Member states use ICAO standards as the basis for their own national aviation regulations to ensure that flights between countries operate safely and consistently.
Context Anchor
Pilots encounter ICAO in international flight planning, airport identifiers, aircraft type codes, aviation documents, and references to worldwide aviation standards.
Derivation
From the words in the name. 'Civil' (from Latin civilis, 'relating to citizens') means non-military aviation. 'International' signals that the body across nations rather than within one. The name itself tells you the : civil flying, worldwide.