An aircraft warning light is a small to medium sized light which alerts aircraft pilots to the building's presence in cases of darkness, fog, rain, or snow. It is most often placed at the top of a structure, but it is sometimes placed on the side when there is a risk that aircraft might fly past the structure below the top level. These lights occur most often on the tallest structures in an area, and they are often mandatory on narrow structures such as television towers and radio transmitters.

The light serves visibility to pilots rather than general illumination of the site. A side-mounted light can warn of an obstruction below the structure's highest point.

A decorative architectural lantern has a different purpose. The warning light is a utility feature even when it occupies the highest point.