A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses or, in older times, from a fire, and used as an aid to navigation for maritime pilots at sea or on inland waterways.

Lighthouses mark dangerous coastlines, hazardous shoals, reefs, safe entries to harbors, and can also assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and replacement by modern electronic navigational devices.

In a lighthouse, the source of light is called the "lamp" (whether electric or fueled by oil) and the concentration of the light is by the "lens" or "optic". Originally lit by open fires and later candles, the Argand hollow wick lamp and parabolic reflector was developed around 1781 in Europe. In the U.S., whale oil was used with wicks as the source of light until the Argand parabolic reflector system was introduced around 1810 by Winslow Lewis. Colza oil replaced whale oil in the early 1850s, but U.S. farmers' lack of interest in growing this caused the service to switch to lard oil in the mid 1850s. Kerosene started replacing lard oil in the 1870s and the lighthouse service was finally converted by the late 1880s. Electricity and carbide (acetylene gas) began replacing kerosene around the turn of the 20th century. It was promoted by the Dalén light, which automatically lit the lamp at nightfall and extinguished it at dawn.