1) Mailing-card 2) Post card 3) Postal card 4) Postcard 
In philately a lettercard or letter card is a postal stationery item consisting of a folded card with a prepaid imprinted stamp. The fact that it is folded over gives the writer twice as much room for the message compared with a postal card. The message is written on the inside and the card is then folded and sealed around the edges. The recipient...
[n] - a postcard that folds so the message is inside
noun a postcard that folds so the message is inside