1. Procedures that weaken an agent of disease (a pathogen): "An attenuation of a virus is a weakened, less vigorous virus."

"A vaccine against a viral disease can be made from an attenuation, a less virulent strain of the virus, a virus capable of stimulating an immune response and creating immunity but not causing illness."

2. In physics, a reduction in amplitude, density, or energy resulting from friction, absorption, or scattering.
3. A loss of solar irradiance as it passes through the atmosphere to the surface of the earth.
4. A reduction in the toxicity of a pollutant substance in the environment; especially, as a result of natural processes: "The attenuations of bacteria and viruses are made less virulent by being heated, dried, treated with chemicals, passed through another organism, or cultured under unfavorable conditions."

"Attenuated bacteria are often used as vaccine agents."

5. The process of reduction: "There is an attenuation of an x-ray beam when its intensity is reduced."
6. The reduction of amplitude, magnitude, or strength of an electrical signal: "In electronics, an attenuation is the opposite of an amplification."

The depletion of electromagnetic energy in the layers of gases surrounding the surface of the Earth because of absorptions or diffusions: The astronomers discovered that the rate of the atmospheric attenuation of the electric sound waves had decreased when passing through dense clouds.