2. A letter designed for airmail consisting of a single sheet of lightweight paper that, once written on, can be folded and sealed to form an envelope; air letter: Jenny used to use aerograms to send letters to her mother, but now she uses emails instead!
The dated term aerogram can also be described as a wireless message.
At one time, it was a telegram and part of its journey was accomplished by an aeroplane (airplane).
3. A message sent "through the air": It used to be that an aerogram was sent by the way of radio!