adjective
1.
(especially in the past) connected with the cheapest and least comfortable way of travelling on a train, ship, etc.
2.
(in the US) connected with the class of mail used to send advertisements, etc.
3.
[ only before noun ] used to describe the lowest standard of degree given by a British university
4.
( disapproving ) ( of people ) less important than other people :
They are treated as third-class citizens.
► ˌthird ˈclass adverb :
to travel third class