WordNet (r) 3.0 (2006):

first class
    adv 1: by first class conveyance; with first class
           accommodations; "we always travel first class"
    n 1: the highest rank in a classification
    2: mail that includes letters and postcards and packages sealed
       against inspection [syn: first class, 1st class, first-
       class mail, 1st-class mail]
    3: the most expensive accommodations on a ship or train or plane