Definition
Tall structures or terrain features along a flight path -- such as towers, antennas, buildings, ridges, or mountains -- that extend high enough to pose a collision hazard to aircraft and that must be cleared by an appropriate margin during cruise flight.
Plain English
Things along the way that stick up high enough to hit -- like towers, hills, or tall buildings -- that the pilot has to fly safely over or around.
Context Anchor
You encounter this term in route planning, night flying, low-altitude operations, and discussions of aircraft lighting and visibility.
Derivation
En route' comes from French, meaning 'on the way.' 'Obstruction' comes from Latin obstruere, 'to build against' or 'block.' Together the phrase means things that get in the way during the journey -- which is exactly how pilots use it: hazards encountered between departure and destination.
Why Pilots Care
Pilots must select altitudes that clear these objects to avoid collisions, particularly at night or in reduced visibility.
Intuition Check
Do not read “en route” as meaning only on a published airway or long cross-country flight. Here it means along the path you are flying between where you depart and where you intend to go.
Example Sentence 1
Before the night cross-country, she studied the chart for en route obstructions and chose a cruise altitude that gave her a comfortable margin above the tallest tower along her route.
Example Sentence 2
Minimum en route altitudes are published to keep aircraft safely above en route obstructions in instrument conditions.