A vertical bar on the right side of a window or a horizontal bar at the bottom of a window that is used to move the window contents up and down or left and right. The bar contains a "thumb", which looks like an elevator in a shaft. When dragged with the mouse, the window contents move correspondingly. When the bar is clicked above or below the thumb, the contents are moved a page at a time. The arrows are clicked to move one line at a time.
Becoming Somewhat Obsolete
Since the advent of scroll mice, which use a wheel for vertical and horizontal scrolling, the scroll bar is being used less and less on desktop applications. In addition, tablet and smartphone content is scrolled by fingers, and a scroll bar was never a natural part of the interface. See scroll mouse
Clicking in the bar or dragging the thumb moves the contents within the window. The Mac example (right) shows an optional variation that places both scroll arrows in the same vicinity to lessen mouse movement when switching directions.