A mouse with a rubber or plastic wheel located between the left and right buttons (the "scroll wheel"). Also known as a "wheel mouse," when the wheel is moved back and forth, the active window is scrolled, eliminating the need to aim the pointer at the scroll bar.
Introduced in 1996, the scroll mouse is very useful for viewing long documents and Web pages, which have a vertical (portrait) orientation. Along with the regular left and right buttons, scroll mice may have side buttons that can be programmed for Previous Page, Next Page and other actions. In addition, the scroll wheel may move from side to side for horizontal scrolling. See touch scroll
and scroll bar
Popularized by Microsoft's IntelliMouse, scroll mice have become the norm ever since. In these examples, the arrows point to the scroll wheels.