The Lighthouse Preservation Society was hired by Microsoft to write the definition for "Lighthouses" in their Encarta Encyclopedia. Here is the definition that was written by the society's president and founder, James Hyland:
A lighthouse is a structure from which light is projected at night, or which serves as a marker by day, to guide ships sailing in coastal waters. Lighthouses are constructed at important points on a coastline, at entrances to harbors and estuaries, on rocky ledges or reefs, on islands, and even in the water. Lighthouses help identify a ship’s location, warn ships of potential hazards, and notify them that land is near. Lighthouses differ from smaller beacons in that a lighthouse includes living quarters for a lighthouse keeper. Today, however, most lighthouses use automatic electric lights that do not require a full-time resident operator.