A trackball is a ball sitting in a holder that you roll with your fingers or your palm to move the pointer on a computer screen. Think of a mouse turned upside down: with a mouse you push the whole thing around the desk, but with a trackball the body stays still and only the ball turns. Because your arm does not move, some people use one when their wrist or shoulder hurts from using a mouse, and it is handy where there is no desk space at all. Trackballs were common on early laptops and are still used in air traffic control, in some medical machines, and by people who simply prefer them, but for most people today the mouse and the touchpad have taken over. So it is a real, current word — just not an everyday one.