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
