Turbo key
The Turbo, Fast Repeat or (simply) Repeat key or button is a modifier key that makes key repeat when held down, or modifies repeat rate from slow to fast.
Focus keyboards
The Turbo key on some keyboards from Focus Electronic is a modifier used in conjunction with the function keys to modify repeat rate for the keyboard.
Keyboards include:
Typewriters
On typewriters, keys such as the Space bar, backspace and Carriage Return don't normally repeat. The Repeat key enables repeat function on some keys: which ones it affects are different from typewriter to typewriter.
Game controllers
A similar Turbo function is found on game controllers, making buttons repeat. There can be a lever for all or a group of buttons, each lever for an individual button or an additional repeating Turbo-button for a button. There is sometimes a rate control. Some levers have multiple steps between Single, Turbo and Autofire, where Autofire in this context means that repeated button press inputs are created without the button needing to be pressed.