What is the Middle Mouse Button?
The middle mouse button (abbreviated MMB) is the mouse button at the middle of the mouse, between the left mouse button and the right mouse button. Typically this will be a scroll wheel that can be pressed, and when pressed it acts like a third, middle button of the mouse.
If a mouse doesn't physically feature a middle mouse button, it may be possible to emulate a middle click by pressing both left and right buttons at the same time.