Definition
A row of in-pavement red lights installed on a taxiway as it approaches a runway, used as part of an airport surface lighting system to warn pilots that entering the runway may be unsafe. When illuminated, the lights indicate that traffic is on or approaching the runway and the aircraft should hold short.
Plain English
Red lights set into the taxiway just before the runway. If they are on, do not enter the runway — something is using it or about to use it.
Context Anchor
You may see REL at taxiway-to-runway intersections at airports equipped with Runway Status Lights.
Why Pilots Care
They provide an immediate visual warning that prevents runway incursions and potential collisions with landing or departing traffic.
Analogy
Think of REL like a red traffic light placed right at the runway entrance. It is there to stop you before you move into a place where crossing traffic may already be present.
Intuition Check
Do not assume REL are runway edge lights or normal guidance lights. REL are red stop-warning lights for runway entry, and they do not replace air traffic control instructions.
Example Sentence 1
As we taxied toward Runway 28R, the Runway Entrance Lights illuminated, so we held short and queried the tower.
Example Sentence 2
The controller cleared us to cross only after confirming the RELs had extinguished.