Definition
The beginning of that portion of a runway usable for landing. It is marked on paved runways by a solid white line across the runway width, and on instrument runways by a row of large white stripes (the threshold bars) at the runway's approach end.
Plain English
The line on the runway where the part you are allowed to land on actually begins. Touching down before this line is landing short of the runway; touching down past it is a normal landing.
Context Anchor
Seen in runway marking, approach, and radar approach discussions when the pilot or controller is referring to where the usable landing part of the runway begins.
Derivation
Threshold comes from Old English meaning the doorway one steps across to enter a room. The landing threshold is the doorway you cross to enter the runway from the air.
Why Pilots Care
Pilots must identify the landing threshold to select the correct aim point and ensure enough runway remains after touchdown.
Intuition Check
Do not assume the landing threshold is always the very end of the pavement. It is the beginning of the runway area approved for landing, which may be farther down the runway.
Example Sentence 1
On a normal approach, the aircraft crosses the landing threshold at about 50 feet above the runway.
Example Sentence 2
Threshold markings clearly identify the landing threshold even in low visibility conditions.