A postmark is the official mark the post office prints on a letter or a parcel. It usually shows the place and the date — sometimes the time too — when that item went into the mail. You often see it stamped right over the stamp you stuck on, so nobody can use that stamp again. That is the everyday job of a postmark: it is proof the letter really was posted, and from where. People also use it as a verb: to postmark a letter is to put that mark on it. In careful talk it stays a post-office word. In looser talk someone might say “the postmark” when they only mean “the date on the envelope,” and skip the place. It is not the pretty stamp you buy; it is the extra mark the office adds.

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