Definition
An IFR clearance issued by ATC in a shortened form, in which only the parts of the clearance that differ from the route or altitude already filed in the flight plan, or previously issued, are spoken. The pilot is expected to fly the filed route except where ATC has explicitly amended it.
Plain English
Instead of reading back every detail of your flight plan, the controller only tells you what has changed. Anything they don't mention stays the same as what you already filed.
Context Anchor
Seen in instrument clearance examples, especially when a controller says a route is approved “as filed” or gives only the parts of a clearance that have changed.
Derivation
"Abbreviated" comes from the Latin brevis, meaning short. Here it simply means "shortened" — ATC is giving you a shortened version of the full clearance because there's no need to repeat what you already know.
Why Pilots Care
Saves radio time and speeds up departures on routine routes.
Intuition Check
Do not assume “abbreviated” means informal or incomplete. In this context, it means shortened, while still carrying the full authority of an air traffic control clearance.
Example Sentence 1
Ground gave us an abbreviated clearance: cleared as filed, climb and maintain 5,000, expect 9,000 in ten minutes.
Example Sentence 2
We received an abbreviated clearance to our destination because the filed route matched the standard departure.