Plain English Meaning
A second airport you plan to fly to instead, in case you can't land at the one you originally intended.
Full Definition
An airport selected as a backup landing site if the planned destination becomes unusable due to weather, mechanical issues, fuel concerns, or operational restrictions such as a closure. It is chosen during flight planning and must meet suitable runway, weather, and fuel-reserve criteria for the flight in progress.
Context Anchor
Seen in flight planning, weather decisions, fuel planning, and notes about whether an airport is open or closed for use.
Derivation
From the Latin 'divertere' meaning 'to turn aside.' A diversionary airport is the place you turn aside to when the original plan no longer works.
Why Pilots Care
Provides a safe, pre-identified option that satisfies and requirements when the intended airport is closed.