airmail vs send

airmail is a noun but send is not a noun.

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

As verbs, send is a hypernym of airmail; that is, send is a word with a broader meaning than airmail:

Other hypernyms of airmail include mail, post.

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

airmail (verb)	send (verb)
send or transport by airmail	cause to be directed or transmitted to another place
	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