Definition: The onkeydown event is used to define actions to be taken when a key on the keyboard is pressed. An onkeydown event is triggered immediately the key is depressed and before the key is released. This is not a reserved word so you can declare your own variable or function called onkeydown but if you do then you will not be able to use this event.
Examples:
<input type="text" onkeydown="myFunction(this)">