Definition of "postmark" [post•mark]

• 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.

• "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.)"