In French, cachet means a stamp or a seal. On a cover, the cachet is an added design or text, often corresponding to the design of the postage stamp, the mailed journey of the cover, or some type of special event. Cachets appear on modern first-day covers, first-flight covers and special-event covers.

A marking intended to show a stamp has been used and is no longer valid as postage. Modern cancels usually include the name of the original mailing location or a nearby sorting facility and the date of mailing. Most cancellations also include a section of lines, bars, text or a design that prints upon the postage stamp to invalidate it. This part of a cancel is called the killer.

Stamps are "canceled to order,'' usually in full sheets, by many governments. The cancels may be printed on the stamps at the same time that the stamp design is printed. A stamp with a cancel and with full gum is likely a CTO stamp, as CTOs do not see actual postal use. CTO stamps are sold to stamp dealers at large discounts from face value. Most catalogs say whether they price CTO stamps or genuinely used stamps.