send vs airmail

send is not a noun while airmail is a noun.

Word	Noun	Adjective	Verb	Adverb
send	No	No	Yes	No
airmail	Yes	No	Yes	No

As verbs, airmail is a hyponym of send; that is, airmail is a word with a more specific, narrower meaning than send:

Other hyponyms of send include express, register, express-mail.

send (noun)	airmail (noun)
	a system of conveying mail by aircraft
	letters and packages that are transported by aircraft

send (verb)	airmail (verb)
cause to be directed or transmitted to another place	send or transport by airmail
transfer
broadcast over the airwaves, as in radio or television
assign to a station
to cause or order to be taken, directed, or transmitted to another place
transport commercially
cause to go somewhere
cause to be admitted; of persons to an institution