air letter vs aerogramme

air letter and aerogramme both are nouns.

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

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

Other synonyms of air letter include aerogram, airmail letter.

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