postal cover
outside of an envelope or package with an address and typically postage stamps that have been cancelled
Bordeaux Cover
the only postal cover, a letter sheet, with the Red and Blue Mauritius (1847)
ship cover
a postal cover that was mailed aboard a ship
Crash cover
an envelope that has been recovered from an air crash, train wreck, shipwreck or other accident
Apollo insurance covers
mementos from Apollo space missions
philatelic cover
envelope sent through the mail for the purpose of creating a collectible item
rarity (philately)
philatelic item that has only survived in very few copies, usually a rare postage stamp or a rare postal cover