Light pollution refers to excess light, created by human activities, that brightens the night sky enough to hide many stars from observers. For the average person, light pollution means that even on a clear, moonless night, only a few stars may be visible. When a city grows up near an astronomical observatory, light pollution can make the observatory effectively useless.
Unlike most forms of pollution, light pollution isn't persistent: turn the lights off, and the dark sky comes back immediately. Like other pollution, though, it is a side effect of industrial civilization: it comes from sources such as domestic lighting, offices, factories, street lighting, and lit sporting venues.
Light pollution is most severe in the highly industrialised, densely populated areas of the United States and Europe, but even relatively small amounts of light can be important for sensitive applications - most major optical observatories have zones many kilometres in diameter severely restricting light emissions.
Light pollution is not just a concern for astronomers. Light shining into the eyes of pedestrians and drivers can reduce visibility. It also reduces night vision, which takes an hour (or more) to return after exposure to bright lights. Light directed out at the viewer (away from a building) causes areas of deep shadow; much home security lighting actually makes houses easier to be broken into, because intruders have more places to hide
Light pollution can be reduced by shielding street lamps so that they light the street below and not the sky above, and by turning off unneeded outdoor lights: for example, only lighting football stadiums when there are people inside saves energy and helps keep the night sky dark.
(For more information, visit the Light Pollution Awareness Group website at www.maltastro.org , and see the International Dark Sky Association: http://www.darksky.org/)