Definition
The minimum weather conditions — specifically ceiling and visibility — that must be forecast at an airport for it to be legally usable as an alternate destination on an instrument flight plan. Standard alternate minimums are 600-foot ceiling and 2 statute miles visibility for a precision approach, and 800-foot ceiling and 2 statute miles visibility for a non-precision approach, unless the airport publishes non-standard alternate minimums or is marked as not authorized for use as an alternate.
Plain English
The weather forecast at your backup airport has to be at least this good before you're allowed to list it as your alternate on an IFR flight plan.
Context Anchor
Seen during IFR flight planning and in instrument procedure publications when deciding whether a backup airport can be filed as the alternate.
Derivation
Alternate comes from a Latin idea of taking turns or using one thing in place of another. Minimum comes from Latin meaning smallest. In aviation, the term points to the smallest weather conditions allowed for the backup airport.
Why Pilots Care
Failing to meet alternate minimums can make a flight plan illegal and leave the pilot without a usable diversion airport in deteriorating weather.
Intuition Check
Alternate does not just mean any airport you like as a backup. For IFR planning, an alternate must meet specific weather requirements before it can be used on the flight plan.
Example Sentence 1
The forecast at our planned alternate showed a 500-foot ceiling, so it didn't meet alternate minimums and we had to pick a different backup airport.
Example Sentence 2
When the destination weather dropped, the crew diverted to the alternate because conditions there remained above minimums.