Light pollution, also known as photopollution or luminous pollution, is excessive, misdirected, or obtrusive artificial light. Pollution is the adding-of/added light itself, in analogy to added sound, carbon dioxide, etc. Adverse consequences are multiple; some of them may not be known yet. Scientific definitions thus include the following:

• Degradation of photic habitat by artificial light.
• Alteration of natural light levels in the outdoor environment owing to artificial light sources.
• Light pollution is the alteration of light levels in the outdoor environment (from those present naturally) due to man-made sources of light. Indoor light pollution is such alteration of light levels in the indoor environment due to sources of light, which compromises human health.
• Light pollution is the introduction by humans, directly or indirectly, of artificial light into the environment.