Plain English Meaning
A sub-route is a smaller part of a longer published route. Instead of flying the whole thing, you fly just the segment between two specific points along it.
Full Definition
Shorter segments of a larger published route, each defined by its own pair of fixes, that allow ATC to clear an aircraft along only a portion of the full route rather than its entire length. Sub-routes are commonly found within Tower En Route Control (TEC) routes and similar substitute route structures, where a long route is broken into smaller usable pieces between intermediate fixes.
Context Anchor
Seen in instrument flight planning and FAA route guidance when a normal airway or route is affected by an outage, restriction, or other change.
Derivation
From the Latin prefix 'sub-' meaning 'under' or 'a part of,' combined with 'route.' A sub-route is literally a route that sits inside another route — a smaller piece of a bigger published path.
Why Pilots Care
Allows pilots to continue IFR operations safely without complete route replanning.
Intuition Check
Do not read “sub-route” as just a smaller shortcut. In this context, it means a replacement route segment used instead of the normal published route.
Example Sentence 1
ATC issued a clearance along a sub-route of the published TEC route, taking the flight only as far as the intermediate fix rather than to the route's terminus.
Example Sentence 2
Before departure the pilot reviewed available sub-routes in case an enroute navigation aid failed.