Definition
The vertical and lateral separation an aircraft maintains from the ground, mountains, towers, buildings, and other physical hazards along its route or approach path. In IFR operations, terrain and obstacle clearance is built into published altitudes, routes, and procedures, and is also a responsibility of ATC when issuing radar vectors and altitude assignments off published procedures.
Plain English
Keeping the aircraft far enough above and away from the ground and anything sticking up from it -- mountains, towers, antennas, buildings -- so there is no risk of hitting them.
Context Anchor
Seen in instrument departure, arrival, approach, and radar-vectoring discussions, especially when ATC assigns headings or altitudes near airports or high ground.
Derivation
Terrain comes from a Latin word meaning earth or land. Obstacle comes from Latin words meaning something that stands in the way. Clearance comes from clear, meaning free from obstruction. Together, the phrase points to keeping the aircraft safely free of the land and objects in its path.
Why Pilots Care
Prevents controlled flight into terrain accidents when pilots are flying on instruments and cannot see the ground.
Grounding Statement
The key idea is simple: the airplane must stay high enough, or in a protected path, so the ground and objects below are not a collision risk.
Intuition Check
Do not read clearance here as only an ATC permission. In this phrase, clearance means safe physical space between the aircraft and terrain or obstacles.
Example Sentence 1
The minimum en route altitude on that airway provides terrain and obstacle clearance of at least 1,000 feet, or 2,000 feet in mountainous areas.
Example Sentence 2
Minimum vectoring altitudes are calculated to guarantee terrain and obstacle clearance for aircraft being guided by radar.