Definition
The beginning of that portion of the runway usable for landing. The threshold is marked on the runway surface and identifies where the landing distance available begins.
Plain English
The line across the runway that marks where you are allowed to start your landing roll. Anything before that line is not counted as usable landing surface.
Context Anchor
Seen on runway markings, airport diagrams, landing distance information, and airport notices that describe runway conditions.
Derivation
From Old English 'þerscold,' meaning the doorway or entry point of a building — the place you cross to go inside. In aviation, the runway threshold is the entry point of the usable landing surface.
Why Pilots Care
Pilots reference the threshold to calculate landing distance available and to establish the correct aiming point on approach.
Intuition Check
Do not assume threshold just means the physical end of the runway. In aviation, it means the official start of the runway portion you may use for landing.
Example Sentence 1
The pilot crossed the threshold at 50 feet and touched down within the first third of the runway.
Example Sentence 2
Runway threshold markings show the start of the available landing distance.