An aerogramme is a very light letter made to go by airmail. The paper is thin; you write on it, then fold it so the sheet itself becomes the envelope. There is no heavy cover, which kept the weight — and the postage — down. Though actually, this is a rather old-fashioned object. Before cheap calls and email, people used aerogrammes to write home from another country. Postal services may still sell them. British spelling keeps the -me (aerogramme); American English often writes aerogram. Some people just say air letter. It is not a telegram, and it is not any letter that happens to go on a plane — it is this special fold-up sheet.