What is Attenuation?

Legal Definition
In physics, attenuation (in some contexts also called extinction) is the gradual loss in intensity of any kind of flux through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water attenuates both light and sound.

In electrical engineering and telecommunications, attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, and in air (radio waves). Electrical attenuators and optical attenuators are commonly manufactured components in this field.
-- Wikipedia
Legal Definition
The amount of energy lost as it travels. It is proportate to the distance it has to travel usually.