It's called Overview button. I originally came to know about this  from the Screen Pinning settings' description.

Google on [Get around on your device](https://support.google.com/nexus/answer/6073614) (see *Get around with navigation buttons*) has noted:

**Overview**: Opens a list of thumbnail images of apps and Chrome tabs you’ve worked with recently. To open an app, touch it. To remove a thumbnail from the list, swipe it left or right. 

* It looks like this

  ![IMG][1]  
* If your device is running Android 4.4 or lower, your Overview button looks like this 

  ![IMG][2]

  [1]: https://i.sstatic.net/7ChcF.png
  [2]: https://i.sstatic.net/RC9zw.png