An Aerogram, Aérogramme or Air Letter is a thin lightweight piece of foldable and gummed paper for writing a letter for transit via airmail, in which the letter and envelope are one and the same. Most postal administrations forbid enclosures in these light letters, which are usually sent abroad at a preferential rate. Printed warnings existed to say that an enclosure would cause the mail would go at the higher letter rate.
Most aerograms have an imprinted stamp indicating the prepayment of postage. As such, this meets the definition of being postal stationery.