aerogramme vs airmail letter

aerogramme and airmail letter both are nouns.

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

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

Other synonyms of aerogramme include aerogram, air letter.

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