1 N-COUNT （门上的）信件投递孔；（建筑物入口处的）信箱 A letterbox is a rectangular hole in a door or a small box at the entrance to a building into which letters and small parcels are delivered. （门上的）信件投递孔；（建筑物入口处的）信箱 [mainly British]
in AM, usually use 美国英语通常用 mailbox
2 ADJ （电视或计算机显示）信箱格式的，上下黑边的，宽屏的 If something is displayed on a television or computer screen in letterbox format, it is displayed across the middle of the screen with dark bands at the top and bottom of the screen. （电视或计算机显示）信箱格式的，上下黑边的，宽屏的