First-class and First class

First-class adjective - Of the very best kind.

First class and first-class are semantically related In some cases you can use "First class" instead the phrase "First-class" as a noun or an adjective.

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

First-class and first class are semantically related in highest in importance topic. Sometimes you can use "First-class" instead the phrase "First class" as an adjective or a noun, if it concerns topics such as highest in rank, most excellent, highest quality, high-ranking.