The one with icons you use to start programs. Probably "main menu."

Ahh! On Android that's called the launcher. The part where you can put widgets and shortcuts is the home screen and where your apps are listed is the app drawer (it literally used to be a drawer.) On iOS it's SpringBoard or home screen.

I can only speak about the former, but If it rotated it would screw up the layout pretty badly as normally the screen is taller than it is wide, so widgets would be truncated and icons wouldn't fit either.