A tower or building erected on headlands along the coast, and on rocks in the sea and in rivers, and at the entrance to harbors, from which a light is shown at night to guide mariners in navigating ships, and to warn them of hidden reefs or dangerous shores. Lights are fixed, revolving, flashing, colored, and intermittent. There are 530 lights round the British coasts, and they are so placed that at any point a ship is always in sight of a light. About 2,000 lights are on the United States coasts.