Definition
A route for which no minimum altitude is published or charted for pilot use. It may, however, be a route for which a minimum vectoring altitude is established for use by ATC when radar service is provided to an aircraft.
Plain English
A flight path that does not appear on any aeronautical chart with a published safe altitude. Pilots cannot fly it on their own using charts — ATC must guide the aircraft along it using radar, with controllers applying their own internal minimum altitudes.
Context Anchor
Seen in IFR routing, air traffic control clearances, and discussions of minimum altitudes along routes that are not printed as standard charted routes.
Derivation
Unpublished' simply means 'not printed or made available to the public.' In aviation, charts are the published reference pilots rely on. If a route is unpublished, it is not on those charts — so the pilot has no charted altitude to use, and the responsibility for terrain and obstacle clearance shifts to the controller.
Why Pilots Care
On an unpublished route, the pilot does not have a charted minimum altitude to fall back on. Terrain and obstacle clearance depends entirely on ATC vectoring the aircraft at or above the controller's minimum vectoring altitude. If radar contact is lost, the pilot must follow lost-communication procedures rather than continue on the unpublished route as if it were charted.
Intuition Check
Do not read unpublished as meaning secret or unofficial. Here it means the route is not shown with a published minimum altitude on the charts pilots normally use.
Example Sentence 1
After departure, Center vectored the aircraft along an unpublished route direct to the fix, keeping it at or above the minimum vectoring altitude.
Example Sentence 2
The pilot accepted the unpublished route after verifying the GPS could navigate the points.