An official mark printed over a postage stamp, especially one that cancels the stamp and records the date and place of mailing.(noun)
To stamp with such a mark.(verb-transitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "postmark" in a sentence
"Another round of phone calls the next week got me as far as getting a rep smart enough in the law to admit that the postmark is what mattered, not the date of arrival, which was a good 5 or 6 days before the due date, but of course they did not retain envelopes as evidence of when I actually sent the cancellation notice."
"And finally, I managed to nearly finish my postcards for International Women's Day - all that remains to do is the satin stitch around the edges, and they can go off in the post this afternoon, with a March 8th postmark, which is appropriate, if slightly later than I have hoped to have them posted."
"(In fact, the postmark was a requirement -- although such ballots were also permissible if they were signed and dated no later than the date of the election.)"