Plain English Meaning
The rules that tell a pilot when they must name a backup airport on an IFR flight plan, and how good the weather has to be at that backup airport for it to count.
Full Definition
The set of regulatory conditions under 14 CFR 91.169 that determine when an IFR flight plan must list an alternate airport, and the weather minimums that airport must meet to be legally usable as that alternate. In general, an alternate must be filed unless the destination has an instrument approach procedure and the forecast weather at the destination, from one hour before to one hour after the estimated time of arrival, calls for a ceiling of at least 2,000 feet above the airport elevation and visibility of at least 3 . When required, the filed alternate must have forecast weather at the ETA that meets either the (600-2 for a , 800-2 for a ) or any non- published for that airport.