Plain English Meaning
A pair of green light bars at the runway threshold, set out to the sides, that help pilots see where the runway begins.
Full Definition
Two transverse rows of green threshold lights extending outward from each side of the runway threshold, used as part of certain approach lighting systems (notably ALSF-2 and SSALR) to mark the start of the landing surface for aircraft approaching from the opposite direction during circling or downwind operations.
Context Anchor
Seen in approach lighting discussions and during final approach when using visual slope lights beside the runway.