postmark
noun
[ ˈpəʊs(t)mɑːk ]
• an official mark stamped on a letter or other postal package, giving the place, date, and time of posting, and serving to cancel the postage stamp.
• "the package had a York postmark"
postmark
verb
• stamp (a postal package) with a postmark.
• "the letter was postmarked New York"
