Hamburger is the slightly tongue-in-cheek term used to refer to the menu button in many applications, particularly mobile applications. The reason for the name is fairly simple – the menu button looks somewhat like a hamburger.
Clicking (or tapping) on the hamburger icon in an application or on a web site typically displays or exposes a menu of items from which to select.
The hamburger is often used when screen size is small or screen real estate is limited, as an alternative to displaying a complete menu or menu bar.
See also: ellipsis.