Commonly seen on mobile apps, the button consisting of three parallel lines is popularly known as what?

The hamburger button, so named for its unintentional resemblance to a hamburger, is a button typically placed in a top corner of a graphical user interface. Its function is to toggle a menu (sometimes referred to as a hamburger menu) or navigation bar between being collapsed behind the button or displayed on the screen. The icon which is associated with this widget, consisting of three horizontal bars, is also known as the 'collapsed menu icon'.

The icon was originally designed by Norm Cox as part of the user interface for the Xerox Star, introduced in 1981; it saw a resurgence starting in 2009 stemming from the limited screen area available to mobile apps. Cox described the icon's creation, saying “Its graphic design was meant to be very “road sign” simple, functionally memorable, and mimic the look of the resulting displayed menu list. With so few pixels to work with, it had to be very distinct, yet simple. I think we only had 16×16 pixels to render the image. (or possibly 13×13… can’t remember exactly)."