Definition of "first class"

first class

adjective
not comparable
1. Belonging to the best or top group in a system of classification.
  Quotations
  The admirable smoothness of the riding also reflected the greatest credit on those who, despite the difficulties caused by the shortage of men and materials, have succeeded in maintaining the track in such first-class order.
  1946 July and August, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 213
2. Of, or relating to the most luxurious and expensive class of accommodation on a train, ship, hotel, etc.
3. Of, or relating to a class of mail to be delivered before second class.
4. (UK, India) Great, very enjoyable.
5. (cricket) Of, or relating to a first class match.
6. (programming) Of, or relating to a treatment like that for a first-class object.
noun
plural first classes
1. (A status, section, etc which is of) premier rank.
  Quotations
  In comparing these services with the exclusive trains-de-luxe of former days, however, it must be remembered that when the original first class was abolished on the Continent and second and third class were promoted to first and second respectively, it meant a considerable reduction in first class fares.
  1962 August, Mercury, “The fastest trains on the Continent, 1962: II—Western Germany”, in Modern Railways, page 126
2. (Ireland) the third year of primary school, following after the junior and senior infant levels.
3. (US) One of two classes of city government in the U.S. state of Kentucky, in which cities elect a mayor and aldermen.