What is the offical name of the third on-screen button?

I am searching for the official name of the third (right) android on-screen button.

The left and middle button are clear:

• Left: Back button


• Middle: Home button


• Right: ???

Android 1 - 4.4 Android 5.0 - ?

Please include a reference where the name of the third/right button is stated in your answer.

Answer

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 (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

  
  

  

  
  


• 
  

  If your device is running Android 4.4 or lower, your Overview button looks like this