aerogram vs airmail letter

aerogram and airmail letter both are nouns.

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

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

Other synonyms of aerogram include aerogramme, air letter.

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