Definition
A line of fixed green lights placed across the approach end of a runway, marking where the usable landing surface begins. They identify the threshold to arriving aircraft and are visible from the approach side; from the departure side, the same fixtures show red to indicate the end of the runway available for takeoff or landing in the opposite direction.
Plain English
Green lights stretched across the start of the runway so pilots flying in can clearly see where the runway begins. The other side of the same lights glows red, showing the far end of the runway to anyone landing or taking off in the other direction.
Context Anchor
You see runway threshold lights when approaching an airport at night or in reduced visibility, and in airport lighting discussions in the Airplane Flying Handbook.
Derivation
Threshold comes from the Old English word for the strip of wood or stone at the bottom of a doorway — the line you cross to enter. The runway threshold is the doorway into the landing surface, and these lights mark that doorway.
Why Pilots Care
Correct identification prevents landing short of the runway or misjudging the touchdown zone, which is essential on short, obstructed, or displaced-threshold runways.
Intuition Check
Do not assume runway threshold lights always mark the very beginning of the pavement. They mark the beginning of the area approved for landing, which may be farther down the runway.
Example Sentence 1
On final approach at night, the line of green threshold lights came into view, confirming the touchdown zone.
Example Sentence 2
The runway threshold lights helped the pilot judge the proper touchdown point after crossing the approach lights.