The International Dark-Sky Association defines light pollution as:

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

Light pollution wastes energy, affects astronomers and scientists, disrupts global wildlife and ecological balance, and has been linked to negative consequences in human health. Read on to learn more about specific issues.

From the International Dark-Sky Association's website