Post·mark n. The mark, or stamp, of a post office on a letter, giving the place and date of mailing or of arrival.
Post·mark, v. t. [imp. & p. p. Postmarked p. pr. & vb. n. Postmarking.] To mark with a post-office stamp; as, to postmark a letter or parcel.
n : a cancellation mark stamped on mail by postal officials;
indicates the post office and date of mailing
v : stamp with a postmark to indicate date and time of mailing