inverse-square law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The principle in physics that the effect of certain forces, such as light, sound, and gravity, on an object varies by the inverse square of the distance between the object and the source of the force. For example, an object placed three feet away from a light source will receive only one ninth ( ¹/_3² , the inverse of 3 squared) as much illumination as an object placed one foot from the light.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics any physical law, such as that of gravitation, in which a quantity varies inversely with distance from a source as the square of that distance