A modifier key is a special key (or combination) on a computer keyboard that temporarily modifies the normal action of another key when pressed together.
Per Modifier Key:
In computing, a modifier key is a special key (or combination) on a computer keyboard that temporarily modifies the normal action of another key when pressed together. By themselves, modifier keys usually do nothing; that is, pressing any of the Shift, Alt, or Ctrl keys alone does not (generally) trigger any action from the computer.
For example, in most keyboard layouts the Shift key combination ⇧ Shift+A will produce a capital letter "A" instead of the default lower-case letter "a" (unless in Caps lock or Shift lock mode). A combination of 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 ⇧ Shift or Alt will probably do nothing unless assigned a specific function in a particular program (for example, activating input aids or the toolbar of the active window in Windows).
