Definition
The beginning of the portion of the runway that is usable for landing. It is marked by a row of white threshold stripes painted across the runway, and on instrument runways it is also indicated by green threshold lights when viewed from the approach end.
Plain English
The line across the runway where the part you can land on starts.
Context Anchor
Pilots use the runway threshold as a visual reference during approach and landing, especially when judging where the airplane should cross the runway end and where touchdown should occur.
Derivation
"Threshold" comes from Old English and originally meant the piece of wood or stone at the bottom of a doorway — the line you cross to enter a building. The runway threshold is the same idea: the line you cross to enter the landing portion of the runway.
Why Pilots Care
Identifying the threshold ensures the correct landing point and helps pilots account for any displaced thresholds that reduce available runway length.
Intuition Check
Do not assume the runway threshold is always the very first piece of pavement. It means the start of the runway area available for landing, which may be farther down the runway if the threshold is displaced.
Example Sentence 1
The pilot aimed to cross the runway threshold at 50 feet, then reduced power to begin the flare.
Example Sentence 2
Runway markings clearly show the threshold so pilots can judge their landing point accurately.