Definition
A pair of synchronized white flashing lights installed on each side of the runway threshold, used to provide rapid and positive identification of the approach end of a particular runway.
Plain English
Two bright strobe lights, one on each side of the start of the runway, that flash together to help pilots quickly spot exactly where the runway begins.
Context Anchor
Seen on airport diagrams, runway lighting notes, and during night or low-visibility approaches to a runway.
Derivation
The name describes the function: lights that identify the end of the runway. 'Identifier' comes from Latin 'identicus' (the same), meaning something that confirms what you are looking at — in this case, confirming you have found the correct runway threshold among other lights at or near the airport.
Why Pilots Care
They give immediate visual confirmation of the runway end, helping prevent misalignment on final approach.
Intuition Check
REIL are not lights along the whole runway. They are the flashing lights that identify the runway end you are approaching for landing.
Example Sentence 1
On the approach into the small regional airport at night, the flashing REIL made the runway threshold easy to pick out from the surrounding lights.
Example Sentence 2
REIL appear on the airport diagram as two flashing lights at the approach end.