Plain English Meaning
The line on the runway where the landing area starts. Anything before this line is not for landing on.
Full Definition
The beginning of that portion of the runway usable for landing.
Context Anchor
You see the threshold at the approach end of a runway, usually marked by runway threshold markings and supported by runway lights at night.
Derivation
From Old English 'therscold,' meaning the bottom of a doorway — the line you cross when entering. The runway threshold marks the line where the landing portion begins.
Why Pilots Care
It determines the start of available landing distance and is critical for calculating touchdown point and decisions.