What is "first flight cover"

n. (context philately English) a philatelically inspired letter flown on the first flight of a new airline service.

In aerophilately, a branch of philately, a first flight cover, also known by the acronym FFC, is mail that has been carried on an inaugural flight of an airline, route, or aircraft, normally postmarked with the date of the flight often of the arrival destination proving it was actually carried on the aircraft and may have a special flight cachet and/or an arrival postmark. Because many first flight covers are essentially made as collectables they can be considered philatelic mail though others consider them to be postal history.