1. [n]  a cancellation mark stamped on mail by postal officials; indicates the post office and date of mailing
2. [v]  stamp with a postmark to indicate date and time of mailing

POSTMARK is a 8 letter word that starts with P.

1. \Post"mark`\, n.
   The mark, or stamp, of a post office on a letter, giving the
   place and date of mailing or of arrival.
   
   

2. \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.