A marking made by a postal service on a letter, package, postcard or the like, usually indicating the place where and the date and time when the item was received or processed for the first time, and often serving to cancel a postage stamp.
1920, (w), Bulldog Drummond Chapter 1
He looked at the mauve envelope doubtfully, and examined the postmark. "Where is Pudlington, James? and one might almost ask—why is Pudlington? No town has any right to such an offensive name."