Lettercard

In philately a lettercard is an item of postal stationery consisting of a folded piece of card with a imprinted stamp. The message is written on the inside and the card sealed around the edges. The recipient tears off the perforated edges to open the card. The term Letter Card is sometimes used on early aerogrammes, however, they are different items.
The lettercard was introduced in Belgium in 1882 and private issues were used in Great Britain in 1887. The first official British letter card was issued in 1892. In Newfoundland reply lettercards were introduced in 1912 which included a small reply card.
bibliography:
Mackay, James. Philatelic Terms Illustrated. 4th edition. London: Stanley Gibbons, 2003, p.78.