A cancellation (or cancel for short) is a postal marking applied to a postage stamp or postal Stationery indicating that the item has been used. Modern Cancellations are often applied simultaneously with a postmark, for efficiency, and commonly the terms “cancellation” and “postmark” are used interchangeably, if incorrectly. (The confusion arises because of the practice of some postal administrations of applying the postmark directly on the stamp, at the cost of legibility.)
The term “killer” is sometimes used as a more vivid synonym.
A cancellation intended solely to prevent reuse of a stamp is sometimes called an obliteration.