An aerogram, aerogramme, aérogramme, air letter or airletter 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 to go at the higher letter rate.

Airmail is a mail transport service branded and sold on the basis of at least one leg of its journey being by air. Airmail items typically arrive more quickly than surface mail, and usually cost more to send. Airmail may be the only option for sending mail to some destinations, such as overseas, if the mail cannot wait the time it would take to arrive by ship, sometimes weeks. The Universal Postal Union adopted comprehensive rules for airmail at its 1929 Postal Union Congress in London. Since the official language of the Universal Postal Union is French, airmail items worldwide are often marked Par avion, literally: "by airplane".