Middle-Click...Middle-Click...Middle-Click...Middle-Click...Middle-Click That's the wheel, for you guys coming from a two-button-mouse Operating System.
Which OS doesn't support multi-button mice?
Windows mice only had two buttons.
The UNIX OSs used 3 button mice right from the start.
When browsers became more widespread, it became more usual to add a scroll-wheel to the two-button mice. But most Windows users didn't twig that the wheel as well as being scrollable, was also clickable. Probably that was because at that time very little software for Windows had the capability of using a 3-button mouse.
UNIX users had the reverse problem. They were used to their mice not being scrollable, so they were very confused when their middle-clicks were also scrolling the mouse (thus moving the cursor) so they were sometimes clicking on the wrong thing.