Electronic mail and First class

Electronic mail noun - (computer science) a system of world-wide electronic communication in which a computer user can compose a message at one terminal that can be regenerated at the recipient's terminal when the recipient logs in.

First class and electronic mail are semantically related. In some cases you can use "First class" instead a noun phrase "Electronic mail".

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

Electronic mail and first class are semantically related. Sometimes you can use "Electronic mail" instead a noun phrase "First class".