Fortification is a process of stopping the fermentation of wort by adding distilled alcohol (e.g. brandy), and allowing to obtain a strong and sweet final product.
At RateBeer we allow fortified mead and cider, but not fortified beer.
To be eligible, the mead or cider must be partially fermented before the fortification happens; unfermented fortified wort is not allowed (e.g. traditional French Pommeau).
Other usages of the word "fortified", to indicate simply that a different ingredient has been added (e.g. "fortified with lactose") are not examples of alcohol fortification and shouldn’t be added here.