Definition
The threshold is the beginning of the portion of a runway that is usable for landing. It is marked on the runway surface and defines where an aircraft may first touch down during a normal landing.
Plain English
The line at the start of a runway where landings are allowed to begin. Cross it, and you are over the part of the runway you can land on.
Context Anchor
Seen in runway markings, airport diagrams, landing distance discussions, and airport notices that use TH as a short form for threshold.
Derivation
From Old English 'þerscold,' meaning the piece of wood or stone at the bottom of a doorway — the point you cross to enter. The runway threshold serves the same role: the entry point to the landing surface.
Why Pilots Care
Pilots use the threshold to calculate landing distance available and to aim their touchdown point.
Intuition Check
Do not assume threshold means any runway end or any paved area near the runway. In this context, it means the marked beginning of the runway area intended for landing.
Example Sentence 1
The pilot crossed the threshold at 50 feet and touched down in the first third of the runway.
Example Sentence 2
Runway 27 threshold is displaced 500 feet due to construction.