Definition
A high-altitude airway that serves both as a conventional jet route, defined by VOR navigation aids, and as an RNAV (area navigation) route, defined by waypoints. Aircraft equipped for VOR navigation can fly the route using the ground-based stations, while RNAV-equipped aircraft can fly the same route using their onboard navigation system and the published waypoints.
Plain English
One published route in the sky that two different kinds of aircraft can use: older ones that follow ground radio stations, and newer ones that follow GPS-style waypoints. Same path, two ways of flying it.
Context Anchor
Seen on IFR En Route High Altitude Charts when a Jet route and an RNAV route are drawn together along the same segment.
Derivation
"Joint" because the single route is shared by two navigation methods. "Jet route" refers to high-altitude airways above 18,000 feet defined by VORs. "RNAV" stands for Area Navigation -- flying directly between any defined points rather than from one ground station to another.
Why Pilots Care
Allows both older and modern aircraft to share the same efficient routing without needing separate airways.
Analogy
It is like one road carrying two highway numbers at the same time. The pavement is the same, but your directions still tell you which route number to follow.
Intuition Check
Joint does not mean a new third kind of route. It means a Jet route and an RNAV route share the same charted path.
Example Sentence 1
We filed the joint jet/RNAV route at FL350, planning to use our GPS waypoints rather than tuning each VOR along the way.
Example Sentence 2
ATC assigned the joint Jet/RNAV route to accommodate both the legacy navigation equipment and the RNAV-equipped traffic.