In computer terms a mouse wheel refers to a scrollable button, in the shape of a wheel, on a computer mouse. The mouse wheel is usually located in the middle of the mouse and can be rolled or scrolled up or down by using the middle or index finger of the hand holding the mouse.
It is mainly used to scroll movable content on a computer screen up or down. It can also be assigned to perform other functions. In some instances the mouse wheel can also be depressed or tilted, acting as another mouse button.