n. 1. The third best or highest group; the class next after the second class. Mary won the pie-making contest in the third class, for the youngest girls. 2. Mail that is printed, other than magazines and newspapers that are published regularly, and packages that are not sealed and weigh less than a pound. The company uses third class to mail free samples of soap. 3. The least expensive class of travel. I couldn't afford anything better than the third class on the ship coming home from France.
Compare: FIRST CLASS, SECOND CLASS.