Light pollution is defined as the introduction of artificial light to an ecosystem (into naturally dark situations), degrading its natural state with the emission of light of different intensities, directions, spectral range and/or unnecessarily varying a light/dark schedule (ie. having lights on a empty office building or streets lights on after certain times).
It is obvious that humans need and use artificial light during the night just…well…simply to live. But the use of artificial light should be in equilibrium with the nocturnal life, energy efficiency and health (avoiding the alteration of sleep patterns and the effect of artificial light at night on some cancers). We should try to minimize the impact of light pollution reducing the negative effects by designing better street lighting, scheduling of street lights, etc.
Light pollution increases the background brightness of the sky by reflecting and diffusion artificial light into the gases and particles in the air. This results in a reduction of visibility of the stars and other celestial objects.
The Sky Law in the Canary Islands (1989) controls gas emissions to the atmosphere on the island, as well as light pollution and air communications routes. The aim of the law is to have transparent and dark sky, with good (if not outstanding) quality for both amateur and professional astronomy.