A key on a computer's keyboard that is only used in conjunction with another key is referred to as a modifier key. Modifier keys are often used with other keys to perform computer keyboard shortcuts and other commands. For example, in many text editor programs pressing the Ctrl + A will select all text.
Examples of modifier keys
On an IBM compatible computer, modifier keys include Alt, Ctrl, Shift, and Windows. On the Apple Macintosh computer, the Control, Option, Command, and Shift keys are modifier keys. Additionally, most laptop and some desktop keyboards contain a Fn modifier key.