Plain English Meaning
A backup place to land — either because plans changed (diversion) or because something has gone wrong and you need to get on the ground (emergency).
Full Definition
An airport, runway, or landing site that a pilot identifies in advance as a usable place to land if the planned destination becomes unavailable or if an in-flight problem requires landing sooner than planned. In the context of closed indefinitely airports, it refers to using such a closed field only when no safer alternative exists and the situation justifies the risk.
Context Anchor
Seen in instrument procedure discussions about airports that are closed indefinitely and should not be treated as normal planned destinations, but may still be considered only in an unplanned safety situation.