Cachet

In philately, a cachet is a printed or stamped design or inscription, other than a cancellation or pre-printed postage, on an envelope, postcard, or postal card to commemorate a postal or philatelic event. There are official and private (independent of postal authorities) cachets. They commemorate everything from the first flight on a particular route, to the Super Bowl. Cachets are also frequently made, either by private companies or a government, for first day of issue stamp events or "second-day" stamp events. They are often present on event covers.