Cachet from "ask phil"
http://www.askphil.org/Cachet: 1: a rubber stamp or printed impression on an envelope which describes the event for which the envelope was mailed; cachets are used for first days of issue, first flights, naval events, stamp exhibitions, etc. 2: rubber stamp or seal, not postal. 3: small marks made by dealers, experts on backs of stamps as marks of authenticity or identification. 4: can be privately applied or officially applied by a post office; also known as "Signum."
Cachet à Date: (Fr.) date stamp.
Cachet à Date Circulaire: (Fr.) circular date stamp.
Cachet à Main: (Fr.) handstamp, a hand-held device for printing that is struck on an ink pad, and then applied to paper.
Cachet au Dos: (Fr.) backstamp; postmark applied to back of incoming mail to show date and time of receipt at the receiving post office.
Cachet Circulaire: (Fr.) circular cancellation.
Cachet de Bord: (Fr.) on board cancel.
Cachet de Cîre: (Fr.) seal (wax).
Cachet de Fantaisie: (Fr.) fancy cancellation.
Cachet de Fortune: (Fr.) improvised cachet.
Cachet de Garantie: (Fr.) French proprietary stamp guaranteeing that goods with stamp affixed are genuine.
Cachet de la Localite: (Fr.) town postmark.
Cacheted Cover: an envelope bearing a type of decoration, tied in to the design of the stamp or a special event.
Cacheté(e): (Fr.) sealed.
Cachet en Bois: (Fr.) wooden hand stamp.
Cachet en Caoutchouc: (Fr.) rubber handstamp.
Cacheter: (Fr.) to seal.
Cachet Faux: (Fr.) forged cancel.
Cachet Maker: someone who designs and produces cachets, either for sale or for personal use
Cachet Manuel: (Fr.) hand cancel.
Cachet Méchanique: (Fr.) machine cancel.
Cachet Muet: (Fr.) special cancellation, temporary.
Cachet Postale: (Fr.) postmark.
Cachet Rond: (Fr.) circular cancellation.
Cachet Spécial: (Fr.) special cancellation.