In philately, a cachet is a picture or design, other than a cancellation or pre-printed postage on the envelope, postcard, postal card or other cover, that can be purely decorative, or commemorative. There are official and private 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 covers.
Cachets have been made by practically every method by which artwork can be made or reproduced. Cachets have been hand-painted, printed and computer-generated. Frequently flight cachets (which have also been used in space and on the moon) are rubber-stamped.
Considered solely as French documents, lettres de cachet may be defined as letters signed by the king of France, countersigned by one of his ministers, and closed with the royal seal (cachet).
The lettre de cachet belonged to the class of lettres closes, as opposed to lettres patentes, which contained the expression of the legal and permanent will of the king, and had to be furnished with the seal of state affixed by the chancellor.
Lettres de cachet were abolished by the Constituent Assembly, but Napoleon reestablished their equivalent by a political measure in the decree of the 9th of March 1801 on the state prisons.