Canceled-to-order: "Stamps are "canceled to order,'' usually in full sheets, by many governments.
Cancel: A marking intended to show a stamp has been used and is no longer valid as postage.
Catalog: A comprehensive book or similar compilation with descriptive information to help identify stamps.
Catalog value: The value of a stamp as listed in a given catalog for the most common condition in which the stamp is collected.
Censored mail: A cover bearing a handstamp or label indicating that the envelope has been opened and the contents inspected by a censor.
Centering: The relative position of the design of a stamp in relation to its margins.
Chalky paper: A chalk-surfaced paper for printing stamps.
Charity stamp: see Semipostal.
Cinderella: A stamplike label that is not a postage stamp.
Classic: An early issue, often with a connotation of rarity, although classic stamps are not necessarily rare. A particularly scarce recent item may be referred to as a modern classic.
Cleaning (stamps): Soiled or stained stamps are sometimes cleaned with chemicals or by erasing. The cleaning is usually done to improve the appearance of a stamp. A cleaned stamp can also mean one from which a cancellation has been removed, making a used stamp appear unused.
Cliché: The individual unit consisting of the design of a single stamp, combined with others to make up the complete printing plate.
Coil: Stamps processed in a long single row and prepared for sale in rolls, often for dispensing from stamp-vending and affixing machines.
Combination cover: Cover bearing the stamps of more than one country when separate postal charges are paid for the transport of a cover by each country.
Commatology: Specialized collecting of postmarks.
Commemorative: A stamp printed in a limited quantity and available for purchase for a limited time.
Compound perforations: Different gauge perforations on different sides of a single stamp.
Condition: The overall appearance and soundness of a stamp or cover. Positive condition factors include fresh full color, full original gum on unused stamps, and so on. Damage such as creases, tears, thinned paper, short perforation teeth, toning and so on negatively affect condition.
Corner card: An imprinted return address, generally in the upper-left corner of an envelope, from a commercial, institutional or private source, similar to business card or letterhead imprints.
Counterfeit: Any stamp, cancellation or cover created for deception or imitation, intended to be accepted by others as genuine.
Cover: An envelope or piece of postal stationery, usually one that has been mailed.
Crease: A noticeable weakening of the paper of a stamp or cover, caused by its being folded or bent at some point.
Cylinder: A curved printing plate used on a modern rotary press.
Cancellation: A marking intended to show a stamp has been used and is no longer valid as postage.
Charity stamp: "A stamp sold at a price greater than postal value, with the additional charge dedicated for a special purpose.