Plain English Meaning
Flatter or rolling terrain — anything not officially designated as mountainous — where instrument procedures only need to keep aircraft 1,000 feet above the tallest obstacle, rather than the larger margin used over mountains.
Full Definition
Areas of terrain that do not meet the FAA's mountainous designation, where instrument procedure designers apply a minimum required obstacle clearance of 1,000 feet above the highest obstacle within the specified area. Non-mountainous areas are those not listed as mountainous in 14 CFR Part 95.
Context Anchor
Seen in instrument procedure design discussions, especially where required obstacle clearance changes depending on whether terrain is classified as mountainous or non-mountainous.
Why Pilots Care
The classification directly affects required obstacle clearance values and the resulting published altitudes on .