Source : WordNet®

1st-class mail
     n : mail that includes letters and postcards and packages sealed
         against inspection [syn: {first class}, {1st class}, {first-class
         mail}, {priority mail}]

Source : Free On-Line Dictionary of Computing

1.TR.6
     
         A {control channel protocol} for
        {ISDN}.  It is a national {standard} in Germany but is being
        replaced by {Euro-ISDN}.
     
        (1995-03-27)