What is Postmark?

Legal Definition
A postmark is a postal marking made on a letter, package, postcard or the like indicating the date and time that the item was delivered into the care of the postal service. Modern postmarks are often applied simultaneously with the cancellation or killer that marks the postage stamp(s) as having been used (though in some circumstances there may be a postmark without a killer, and sometimes the postmark and killer form a continuous design), and the two terms are often used interchangeably, if incorrectly. Postmarks may be applied by hand or by machines, using methods such as rollers or inkjets, while digital postmarks are a recent innovation. The local post Hawai'i Post had a rubber-stamp postmark, parts of which were hand-painted. At Hideaway Island, Vanuatu, the Underwater Post Office has an embossed postmark.
-- Wikipedia
Legal Definition
A stamp or, mark put on letters in the post office.

2. Post marks are evidence of a letter having passed through the post office. 2 Camp. 620; 2 B. & P. 316; 15 East, 416; 1 M. & S. 201; 15 Com. R. 206.
-- Bouviers Law Dictionary
Legal Definition
A stamp or mark put on letters received at the post-office for transmission through the mails.
-- Black's Law Dictionary