What is the collective name for the power, volume, and/or function buttons on a tablet?

Solution 1:

Microsoft uses "hardware buttons" and "physical buttons" interchangeably.

Physical buttons

Hardware buttons let users perform many common tasks that do not have a convenient user interface alternative. For the scenarios addressed in this section, the hardware buttons are typically used for tasks that occur while the physical keyboard is not available to the user, on form factors such as convertibles or slates.

You could really use either one. But as always, pick one and stick to it.

Solution 2:

Well, on the iPad user guide by Apple Inc., It tells you the names of the buttons.

On an IPad Pro:

On Ipad(5th generation), IPad Air, IPad Air 2:

Hope it helped.