adjective

1. of the highest class, rank, excellence, etc.; of the best quality
2. designating or of the most expensive accommodations a first-class cabin on a ship
3. ☆ designating or of a class of ordinary mail given privileged postal treatment (as forwarding, if necessary) at postage rates higher than those for other classes of ordinary mail: the class includes letters or other material sealed against inspection and also includes, at a postage rate lower than that for sealed material, postal cards and postcards

adverb

1. with the most expensive accommodations to travel first-class
2. as or by first-class mail