keycode 13 is for which key

Which is the key on the keyboard having the keycode as 13?

switch(key) {
  case 37: 
    $.keynav.goLeft();
    break;
  case 38: 
    $.keynav.goUp();
    break;
  case 39: 
    $.keynav.goRight();
    break;
  case 40: 
    $.keynav.goDown();
    break;
  case 13: 
    $.keynav.activate();
    break;
}