A cancellation is a postal marking applied on a postage stamp or postal stationery to deface the stamp and to prevent its reuse. Cancellations come in a huge variety of designs, shapes, sizes, and colors. Modern cancellations commonly include the date and post office location where the stamps were mailed, in addition to lines or bars designed to cover the stamp itself. The term "postmark" refers specifically to the part that contains the date and posting location, but the term is often used interchangeably with "cancellation" as it may serve that purpose. The portion of a cancellation that is designed to deface the stamp and does not contain writing is also called the "obliteration" or killer. Some stamps are issued pre-cancelled with a printed or stamped cancellation and do not need to have a cancellation added. Cancellations can affect the value of stamps to collectors, positively or negatively. Cancellations of some countries have been extensively studied by philatelists, and many stamp collectors and postal history collectors collect cancellations in addition to the stamps themselves.

A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage. Then the stamp is affixed to the face or address-side of any item of mail—an envelope or other postal cover —which they wish to send. The item is then processed by the postal system, where a postmark or cancellation mark—in modern usage indicating date and point of origin of mailing—is applied to the stamp and its left and right sides to prevent its reuse. Next the item is delivered to its addressee.