aerogramme vs air letter

aerogramme and air letter both are nouns.

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

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

Other synonyms of aerogramme include aerogram, airmail letter.

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