Definition
An airport selected by the pilot, prior to or during flight, as a landing site to be used if an in-flight emergency or abnormal condition makes continuing to the planned destination unsafe or impractical. It is chosen for its suitability under the specific conditions of the flight, considering factors such as runway length, weather, available approaches, fuel remaining, and aircraft performance.
Plain English
A backup airport you've already picked out in case something goes wrong and you need to land somewhere other than where you were originally heading.
Context Anchor
Used during flight planning and in flight when a pilot decides where to land after a serious problem such as worsening weather, low fuel, aircraft trouble, or a passenger medical issue.
Derivation
"Diversion" comes from the Latin divertere, meaning "to turn aside." An emergency diversion airport is one you turn aside to when continuing the original plan is no longer the right call.
Why Pilots Care
The choice directly affects how quickly a safe landing can be made when time is critical.
Intuition Check
Do not assume an emergency diversion airport is always the closest airport. It is the most suitable safe place to land, considering the problem, the aircraft, the weather, and the runway.
Example Sentence 1
Before departure, she identified two emergency diversion airports along the route in case weather at the destination dropped below minimums.
Example Sentence 2
The preflight briefing included identifying at least one emergency diversion airport along the route in case of a medical issue.