Definition
The vertical and horizontal separation maintained between an aircraft and any terrain, structures, vegetation, towers, wires, or other objects that could be struck during flight. In ground reference maneuvers and low-altitude flight, obstruction clearance refers specifically to selecting an altitude and ground track that keep the airplane safely above and clear of such objects throughout the maneuver.
Plain English
Keeping enough room between the airplane and anything you could hit -- trees, towers, wires, hills, or buildings -- both below and around you.
Context Anchor
In drift and ground track control, obstruction clearance matters when wind pushes the airplane sideways from the intended path near the ground.
Derivation
From Latin 'obstruere,' meaning 'to block or build against.' An obstruction is anything standing in the way. 'Clearance' here means the space kept between the airplane and that obstruction -- not permission, as in 'ATC clearance.'
Why Pilots Care
Maintaining obstruction clearance prevents controlled flight into terrain and collisions with obstacles, which remain leading causes of accidents in visual flight.
Intuition Check
Do not read clearance here as permission from air traffic control. Here, clearance means physical space between the aircraft and an obstacle.
Example Sentence 1
Before beginning turns around a point, the pilot chose an altitude that provided adequate obstruction clearance from the surrounding trees and power lines.
Example Sentence 2
Strong winds required frequent heading changes to keep obstruction clearance while following the road at 800 feet AGL.