Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.

The International Dark-Sky Association (IDA) defines light pollution as:

Any adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.

This approach confuses the cause and the result, however. Pollution is the adding-of/added light itself, in analogy to added sound, carbon dioxide, etc. Adverse consequences are multiple; some of them may be not known yet. Scientific definitions thus include the following:

• 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.

The first two of the above three scientific definitions describe the state of the environment. The third (and newest) one describes the process of polluting by light.

Light pollution obscures the stars in the night sky for city dwellers, interferes with astronomical observatories, and, like any other form of pollution, disrupts ecosystems and has adverse health effects. Light pollution can be divided into two main types:

(1) annoying light that intrudes on an otherwise natural or low-light setting and

(2) excessive light (generally indoors) that leads to discomfort and adverse health effects.

Since the early 1980s, a global dark-sky movement has emerged, with concerned people campaigning to reduce the amount of light pollution.

Light pollution is a side effect of industrial civilization. Its sources include building exterior and interior lighting, advertising, commercial properties, offices, factories, streetlights, and illuminated sporting venues. It is most severe in highly industrialized, densely populated areas of North America, Europe, and Japan and in major cities in the Middle East and North Africa like Tehran and Cairo, but even relatively small amounts of light can be noticed and create problems. Like other forms of pollution (such as air, water, and noise pollution) light pollution causes damage to the environment.