In postal use a cancellation is any type of mark made on a postage stamp, the part of a piece of postal stationary such as a stamped envelope or postal card that indicates that postage was paid or how much postage was paid, or perhaps a meter label (though typically a cancellation is not required on a meter label), for the purpose of rendering the postage stamp or other form of indication postage has been paid, invalid and hopefully, from the point-of-view of the post office, unusable, for reuse.
The earliest cancellations were pen cancels.