The en route phase of flight is defined as that segment of flight from the termination point of a departure procedure to the origination point of an arrival procedure. The procedures employed in the en route phase of flight are governed by a set of specific flight standards established by 14 CFR [Figure], FAA Order 8260.3, and related publications. These standards establish courses to be flown, obstacle clearance criteria, minimum altitudes, navigation performance, and communications requirements.

Aircraft En Route Operations
Code of Federal Regulations, Title 14 Aeronautics and Space