Gunning Fog Index
Definition - A method of assessing the readability of texts that was devised by Robert Gunning and described in his book The Technique of clear writing. For example, words with three or more syllables are foggy words.

An example of readability analysis software that incorporates Gunning Fox

1. "The index estimates the years of formal education needed to understand the text on a first reading. A fog index of 12 requires the reading level of a U.S. high school senior (around 18 years old). … The fog index is commonly used to confirm that text can be read easily by the intended audience. Texts for a wide audience generally need a fog index less than 12. Texts requiring near-universal understanding generally need an index less than 8." (Wikipedia)

Example -
The following paragraph has a Gunning Fog Index of 3.1.

Rock-a-bye baby, on the treetop.

When the wind blows, the cradle will rock.

When the bough breaks, the cradle will fall.

And down will come baby, cradle and all.

Please comment