airmail letter vs aerogram

airmail letter and aerogram both are nouns.

Word	Noun	Adjective	Verb	Adverb
airmail letter	Yes	No	No	No
aerogram	Yes	No	No	No

As nouns, airmail letter and aerogram are synonyms defined as:

Other synonyms of airmail letter include aerogramme, air letter.

airmail letter (noun)	aerogram (noun)
a letter sent by air mail	a letter sent by air mail