The Collaborative International Dictionary of English v.0.48:

1st-class \1st-class\ n.
   1. a class mail comprising letters, postcards, and other mail
      sealed against inspection, having a higher priority than
      second, third, or fourth-class mail; -- it is the highest
      class of mail not handled in a special manner, as is
      registered or priority mail.

   Syn: first-class, first-class mail, 1st-class mail
        [WordNet 1.5]