What is "modifier key"

n. (context computing English) A key (of a computer keyboard) that may be pressed simultaneously with certain other keys to activate predefined functions.

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 , , or keys alone does not (generally) trigger any action from the computer.

For example, in most keyboard layouts the Shift key combination 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 in Microsoft Windows will close the program in the active window; in this instance, Alt is the modifier key. In contrast, pressing just or 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).

User interface expert Jef Raskin coined the term " quasimode" to describe the state a computer enters into when a modifier key is pressed.