modifier key

In computing, a modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination.

For example, Alt+F4 in Microsoft Windows will close the program in the active window; in this instance, Alt is the modifier key. In contrast, pressing just F4 will probably do nothing unless assigned a specific function in a particular program. By themselves, modifier keys usually do nothing; that is, pressing Alt alone does not trigger any action from the computer.

refer from:http://en.wikipedia.org/wiki/Modifier_key