A searchlight is a very strong electric light that you can turn to point anywhere. The beam is a bright bar in the dark. People use it to find someone, to watch a prison wall or a harbour, or in war to catch a plane in the sky. You also see them at big night shows, sweeping across clouds. A torch in your hand is a light too, though actually a searchlight is in another league: it is large, often on a truck, a roof, or a ship, and it is meant to reach far. The word is simply search + light — a light you search with. It is mostly a noun; you can use it as a verb (to searchlight a hillside), but that sounds a bit technical or old. In everyday talk, people just say they turned the searchlight on.