Plain English Meaning
A backup airport the pilot can fly to if the planned destination is no longer a safe or workable place to land.
Full Definition
An airport, other than the original destination, that a pilot plans to fly to if landing at the intended destination becomes impractical or impossible due to weather, mechanical issues, traffic, or other operational factors. The alternate is selected before or during flight and must meet specific suitability requirements, including adequate weather minimums and sufficient fuel reserves to reach it after attempting the primary destination.
Context Anchor
Seen during flight diversion planning, especially when weather, fuel, , or other changing require a pilot to a new place to land.